Two related changes to timing code:

First, make absolute times unsigned.  This means that it's safe to
depend on their overflow behaviour (which is undefined for signed
integers).  This requires a little extra care in handling comparisons,
but I think I've correctly adjusted them all.

Second, functions registered with schedule_timer() are guaranteed to be
called with precisely the time that was returned by schedule_timer().
Thus, it's only necessary to check these values for equality rather than
doing risky range checks, so do that.

The timing code still does lots that's undefined, unnecessary, or just
wrong, but this is a good start.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9667 cda61777-01e9-0310-a592-d414129be87e