MEDIUM: connection: make xprt->snd_buf() take the byte count in argument

MEDIUM: connection: make xprt->snd_buf() take the byte count in argument

This way the senders don't need to modify the buffer's metadata anymore
nor to know about the output's split point. This way the functions can
take a const buffer and it's clearer who's in charge of updating the
buffer after a send. That's why the buffer realignment is now performed
by the caller of the transport's snd_buf() functions.

The return type was updated to return a size_t to comply with the count
argument.