MEDIUM: stream-interface: add a snd_buf() callback to sock_ops

MEDIUM: stream-interface: add a snd_buf() callback to sock_ops

This callback is used to send data from the buffer to the socket. It is
the old write_loop() call of the data layer which is used both by the
->write() callback and the ->chk_snd() function. The reason for having
it as a pointer is that it's the only remaining part which causes the
write and chk_snd() functions to be different between raw and ssl.