Retired the #ifdef DUMP_PACKETS stuff in ssh.c because I'm utterly

Retired the #ifdef DUMP_PACKETS stuff in ssh.c because I'm utterly
sick of recompiling to enable packet dumps. SSH packet dumping is
now provided as a logging option, and dumps to putty.log like all
the other logging options. While I'm at it I cleaned up the format
so that packet types are translated into strings for easy browsing.
POSSIBLE SIDE EFFECT: in the course of this work I had to re-enable
the SSH1 packet length checks which it turns out hadn't actually
been active for some time, so it's possible things might break as a
result. If need be I can always disable those checks for the 0.52
release and think about it more carefully later.

[originally from svn r1493]