New formatting directive in logfile naming: &P for port number.

New formatting directive in logfile naming: &P for port number.

Users have requested this from time to time, for distinguishing log
file names when there's more than one SSH server running on different
ports of the same host. Since we do take account of that possibility
in other areas (e.g. we cache host keys indexed by (host,port) rather
than just host), it doesn't seem unreasonable to do so here too.

(cherry picked from commit 0550943b51c538400e31ce18483032e446178120)