hangup revamp (trunk only)

hangup revamp (trunk only)

[See cvs log for src/cmd.c for more complete description.]

     This turns clearlocks() into a no-op during the period when the UNIX
port is asking the user to confirm whether to overwrite an existing game.
Also, this removes the duplication of code and function between hangup()
and end_of_input(), and it simplifies the check for whether hangups are
supported by adding new macro HANGUPHANDLING.  (I don't think global.h is
the best place to be defining that but I couldn't figure out where else
it would fit, other than repeating for individual xxxconf.h files.)  And
adds a couple more done_hup checks to try to cope with situations where
rhack() is being bypassed.  Lastly, having readchar() return EOF was
ignored for non-UNIX configs; now everybody gets ESC instead of letting
EOF be seen further inside the core.