Tidy up signal handling.

Tidy up signal handling.
All signal() calls have been changed to pending_signal() calls.
pending_signal() is defined in the new sig.c file.  It remembers
the handler and traps the signal with a function that will remember
the signal.

main.c now calls handle_signals() to actually call the required
handlers (if the above handler was called).

If this doesn't close PR2662 (was PR2347), I'll cry.

Joerg, I think this should go into 2.2, but I havn't done anything
about it because I'm bound to botch it with the new sig.[ch] files.

I've just "cvs add"'d sig.[ch] so far.... can you update to 2.2 and
tell me what you did ?  Thanks.