g_unix_set_fd_nonblocking: New API to control file descriptor blocking state

And use it in relevant places in GLib.

https://bugzilla.gnome.org/show_bug.cgi?id=649225

glib/glib-unix.h

@@ -61,6 +61,10 @@ gboolean g_unix_pipe_flags (int      *fds,
 			    int       flags,
 			    GError  **error);
 
+gboolean g_unix_set_fd_nonblocking (gint       fd,
+				    gboolean   nonblock,
+				    GError   **error);
+
 GSource *g_unix_signal_source_new     (int signum);
 
 guint    g_unix_signal_add_watch_full (int            signum,