Annotate the return value of various utility functions

While the introspection scanner can glean the transfer rule for the
return values by looking at their constness, adding an explicit
annotation has the advantage of gtk-doc writing out the transfer rule as
an additional bit of documentation, making the life of the documentation
reader easier.

glib/gutils.c

@@ -301,8 +301,8 @@ g_find_program_in_path (const gchar *program)
  * the program is found, the return value contains the full name
  * including the type suffix.
  *
- * Returns: (type filename): a newly-allocated string with the absolute path,
- *     or %NULL
+ * Returns: (type filename) (transfer full) (nullable): a newly-allocated
+ *   string with the absolute path, or %NULL
  **/
 #ifdef G_OS_WIN32
 static gchar *
@@ -735,7 +735,7 @@ g_get_user_database_entry (void)
  * encoding, or something else, and there is no guarantee that it is even
  * consistent on a machine. On Windows, it is always UTF-8.
  *
- * Returns: (type filename): the user name of the current user.
+ * Returns: (type filename) (transfer none): the user name of the current user.
  */
 const gchar *
 g_get_user_name (void)
@@ -756,7 +756,7 @@ g_get_user_name (void)
  * real user name cannot be determined, the string "Unknown" is 
  * returned.
  *
- * Returns: (type filename): the user's real name.
+ * Returns: (type filename) (transfer none): the user's real name.
  */
 const gchar *
 g_get_real_name (void)
@@ -865,7 +865,7 @@ g_build_home_dir (void)
  * should either directly check the `HOME` environment variable yourself
  * or unset it before calling any functions in GLib.
  *
- * Returns: (type filename): the current user's home directory
+ * Returns: (type filename) (transfer none): the current user's home directory
  */
 const gchar *
 g_get_home_dir (void)
@@ -901,7 +901,7 @@ g_get_home_dir (void)
  * it is always UTF-8. The return value is never %NULL or the empty
  * string.
  *
- * Returns: (type filename): the directory to use for temporary files.
+ * Returns: (type filename) (transfer none): the directory to use for temporary files.
  */
 const gchar *
 g_get_tmp_dir (void)
@@ -966,7 +966,7 @@ g_get_tmp_dir (void)
  *
  * The encoding of the returned string is UTF-8.
  *
- * Returns: the host name of the machine.
+ * Returns: (transfer none): the host name of the machine.
  *
  * Since: 2.8
  */
@@ -1046,9 +1046,9 @@ static gchar *g_prgname = NULL;
  * #GtkApplication::startup handler. The program name is found by
  * taking the last component of @argv[0].
  *
- * Returns: (nullable): the name of the program, or %NULL if it has not been
- *     set yet. The returned string belongs
- *     to GLib and must not be modified or freed.
+ * Returns: (nullable) (transfer none): the name of the program,
+ *   or %NULL if it has not been set yet. The returned string belongs
+ *   to GLib and must not be modified or freed.
  */
 const gchar*
 g_get_prgname (void)
@@ -1100,7 +1100,8 @@ static gchar *g_application_name = NULL;
  * g_get_prgname() (which may be %NULL if g_set_prgname() has also not
  * been called).
  * 
- * Returns: human-readable application name. may return %NULL
+ * Returns: (transfer none) (nullable): human-readable application
+ *   name. May return %NULL
  *
  * Since: 2.2
  **/
@@ -1300,8 +1301,9 @@ g_build_user_data_dir (void)
  * Note that in this case on Windows it will be the same
  * as what g_get_user_config_dir() returns.
  *
- * Returns: (type filename): a string owned by GLib that must not be modified
- *               or freed.
+ * Returns: (type filename) (transfer none): a string owned by GLib that must
+ *   not be modified or freed.
+ *
  * Since: 2.6
  **/
 const gchar *
@@ -1360,8 +1362,8 @@ g_build_user_config_dir (void)
  * Note that in this case on Windows it will be  the same
  * as what g_get_user_data_dir() returns.
  *
- * Returns: (type filename): a string owned by GLib that must not be modified
- *               or freed.
+ * Returns: (type filename) (transfer none): a string owned by GLib that
+ *   must not be modified or freed.
  * Since: 2.6
  **/
 const gchar *
@@ -1419,8 +1421,8 @@ g_build_user_cache_dir (void)
  * `C:\Documents and Settings\username\Local Settings\Temporary Internet Files`.
  * See the [documentation for `CSIDL_INTERNET_CACHE`](https://msdn.microsoft.com/en-us/library/windows/desktop/bb762494%28v=vs.85%29.aspx#csidl_internet_cache).
  *
- * Returns: (type filename): a string owned by GLib that must not be modified
- *               or freed.
+ * Returns: (type filename) (transfer none): a string owned by GLib that
+ *   must not be modified or freed.
  * Since: 2.6
  **/
 const gchar *
@@ -2258,8 +2260,8 @@ g_nullify_pointer (gpointer *nullify_location)
  * See g_format_size_full() for more options about how the size might be
  * formatted.
  *
- * Returns: a newly-allocated formatted string containing a human readable
- *     file size
+ * Returns: (transfer full): a newly-allocated formatted string containing
+ *   a human readable file size
  *
  * Since: 2.30
  */
@@ -2297,8 +2299,8 @@ g_format_size (guint64 size)
  * This function is similar to g_format_size() but allows for flags
  * that modify the output. See #GFormatSizeFlags.
  *
- * Returns: a newly-allocated formatted string containing a human
- *     readable file size
+ * Returns: (transfer full): a newly-allocated formatted string
+ *   containing a human readable file size
  *
  * Since: 2.30
  */
@@ -2514,8 +2516,8 @@ g_format_size_full (guint64          size,
  *
  * This string should be freed with g_free() when not needed any longer.
  *
- * Returns: a newly-allocated formatted string containing a human
- *     readable file size
+ * Returns: (transfer full): a newly-allocated formatted string
+ *   containing a human readable file size
  *
  * Since: 2.16
  *