MEDIUM: connection: make xprt->rcv_buf() use size_t for the count

MEDIUM: connection: make xprt->rcv_buf() use size_t for the count

Just like we have a size_t for xprt->snd_buf(), we adjust to use size_t
for rcv_buf()'s count argument and return value. It also removes the
ambiguity related to the possibility to see a negative value there.