MEDIUM: buffers: implement b_adv() to advance a buffer's pointer

MEDIUM: buffers: implement b_adv() to advance a buffer's pointer

This is more convenient and efficient than buf->p = b_ptr(buf, n);
It simply advances the buffer's pointer by <n> and trasfers that
amount of bytes from <in> to <out>. The BF_OUT_EMPTY flag is updated
accordingly.

A few occurrences of such computations in buffers.c and stream_sock.c
were updated to use b_adv(), which resulted in a small code shrink.