Add GCancellables to GSocket ops

Add GCancellables to GSocket ops

Currently, to implement cancellability correctly, all synchronous
calls to GSocket must be preceded by a g_socket_condition_wait() call,
(even though GSocket does this internally as well) and all
asynchronous calls must do occasional manual
g_cancellable_is_cancelled() checks. Since it's trivial to do these
checks inside GSocket instead, and we don't particularly want to
encourage people to use the APIs non-cancellably, move the
cancellation support into GSocket and simplify the existing callers.

http://bugzilla.gnome.org/show_bug.cgi?id=586797