Always do async vs sync correctly in GSocketConnection streams

Previously if a GSocketConnection had a blocking GSocket, it would
sometimes block during asynchonous I/O, and if it had a non-blocking
socket, it would sometimes return G_IO_ERROR_WOULD_BLOCK from
synchronous I/O. This fixes the connection to not depend on the socket
state.

https://bugzilla.gnome.org/show_bug.cgi?id=616458