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

gio/glocalfileinfo.c

@@ -63,6 +63,7 @@
 #include <gvfs.h>
 
 #ifndef G_OS_WIN32
+#include "glib-unix.h"
 #include "glib-private.h"
 #endif
 #include "glibintl.h"
@@ -1260,7 +1261,7 @@ get_content_type (const char          *basename,
 	      ssize_t res;
 	      
 	      res = read (fd, sniff_buffer, sniff_length);
-	      close (fd);
+	      (void) g_close (fd, NULL);
 	      if (res >= 0)
 		{
 		  g_free (content_type);