add a ssh_packet_process_read() function that reads from a fd

add a ssh_packet_process_read() function that reads from a fd
directly into the transport input buffer.

Use this in the client and server mainloops to avoid unnecessary
copying. It also lets us use a more greedy read size without penalty.

Yields a 2-3% performance gain on cipher-speed.sh (in a fairly
unscientific test tbf)

feedback dtucker@ ok markus@