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/gapplicationimpl-dbus.c

@@ -32,6 +32,7 @@
 #include "gdbusconnection.h"
 #include "gdbusintrospection.h"
 #include "gdbuserror.h"
+#include "glib/gstdio.h"
 
 #include <string.h>
 #include <stdio.h>
@@ -709,7 +710,7 @@ g_dbus_command_line_get_stdin (GApplicationCommandLine *cmdline)
       fds = g_unix_fd_list_steal_fds (fd_list, &n_fds);
       result = g_unix_input_stream_new (fds[0], TRUE);
       for (i = 1; i < n_fds; i++)
-        close (fds[i]);
+        (void) g_close (fds[i], NULL);
       g_free (fds);
     }