skype: cleanup of the send() code

Use socket.sendall(), as send() is not guaranteed to send all
the data passed (though it should generally work that way with
blocking sockets).
Use more robust, obvious and idiomatic loop conditions.
Cleanup redundant imports and variables.
skype: don't do retries on send() on socket errors with gobject

As it's a blocking socket, I imagine there might be only EINTR
errors from signals, which don't seem to be used here.
I think the same tweak can be applied to select() code, but
I don't run it, so no idea if Skype4Py threads might make such loop
necessary there.
skype: handle socket errors during tls session negotiation in a graceful way

Before this patch, such errors (which happen 1/4 times here) lock
skyped forever, producing traceback (and hanging because threads
in python).
Proper fix would be to see why these happen (might be ssl handling
in the plugin), but that's no excuse not to handle socket errors
without crashing the daemon.
skype: handle only socket errors in send()