Add g_mkdtemp in the spirit of g_mkstemp

At the same time, also add g_mkdtemp_full and g_dir_make_tmp
variants. The patch also unifies the unique-name-generating
code for all variants of mkstemp and mkdtemp and adds tests
for the new functions.

Based on patches by Paolo Bonzini,
http://bugzilla.gnome.org/show_bug.cgi?id=118563

docs/reference/glib/glib-sections.txt

@@ -1174,6 +1174,9 @@ g_mkstemp_full
 g_file_open_tmp
 g_file_read_link
 g_mkdir_with_parents
+g_mkdtemp
+g_mkdtemp_full
+g_dir_make_tmp
 
 <SUBSECTION>
 GDir