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/glib-unix.c

@@ -179,7 +179,6 @@ g_unix_set_fd_nonblocking (gint       fd,
 #endif
 }
 
-
 /**
  * g_unix_signal_source_new:
  * @signum: A signal number