Add g_close(), use it

There are two benefits to this:

1) We can centralize any operating system specific knowledge of
   close-vs-EINTR handling.  For example, while on Linux we should never
   retry, if someone cared enough later about HP-UX, they could come by
   and change this one spot.
2) For places that do care about the return value and want to provide
   the caller with a GError, this function makes it convenient to do so.

Note that gspawn.c had an incorrect EINTR loop-retry around close().

https://bugzilla.gnome.org/show_bug.cgi?id=682819

glib/gstdio.c

@@ -835,3 +835,46 @@ g_utime (const gchar    *filename,
   return utime (filename, utb);
 #endif
 }
+
+/**
+ * g_close:
+ * @fd: A file descriptor
+ * @error: a #GError
+ *
+ * This wraps the close() call; in case of error, %errno will be
+ * preserved, but the error will also be stored as a #GError in @error.
+ *
+ * Besides using #GError, there is another major reason to prefer this
+ * function over the call provided by the system; on Unix, it will
+ * attempt to correctly handle %EINTR, which has platform-specific
+ * semantics.
+ */
+gboolean
+g_close (gint       fd,
+         GError   **error)
+{
+  int res;
+  res = close (fd);
+  /* Just ignore EINTR for now; a retry loop is the wrong thing to do
+   * on Linux at least.  Anyone who wants to add a conditional check
+   * for e.g. HP-UX is welcome to do so later...
+   *
+   * http://lkml.indiana.edu/hypermail/linux/kernel/0509.1/0877.html
+   * https://bugzilla.gnome.org/show_bug.cgi?id=682819
+   * http://utcc.utoronto.ca/~cks/space/blog/unix/CloseEINTR
+   * https://sites.google.com/site/michaelsafyan/software-engineering/checkforeintrwheninvokingclosethinkagain
+   */
+  if (G_UNLIKELY (res == -1 && errno == EINTR))
+    return TRUE;
+  else if (res == -1)
+    {
+      int errsv = errno;
+      g_set_error_literal (error, G_FILE_ERROR,
+                           g_file_error_from_errno (errsv),
+                           g_strerror (errsv));
+      errno = errsv;
+      return FALSE;
+    }
+  return TRUE;
+}
+