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/gdbusprivate.c

@@ -39,6 +39,7 @@
 #include "ginputstream.h"
 #include "gmemoryinputstream.h"
 #include "giostream.h"
+#include "glib/gstdio.h"
 #include "gsocketcontrolmessage.h"
 #include "gsocketconnection.h"
 #include "gsocketoutputstream.h"
@@ -621,7 +622,7 @@ _g_dbus_worker_do_read_cb (GInputStream  *input_stream,
                     {
                       /* TODO: really want a append_steal() */
                       g_unix_fd_list_append (worker->read_fd_list, fds[n], NULL);
-                      close (fds[n]);
+                      (void) g_close (fds[n], NULL);
                     }
                 }
               g_free (fds);