Render timing.c robust in the face of strangeness. The strangenesses

in question vary per OS: on Windows the problem is that WM_TIMER
sometimes goes off too early, so that GetTickCount() is right and
the callback time is wrong, whereas on Unix the problem is that my
GETTICKCOUNT implementation comes from the system clock which means
it can change suddenly and non-monotonically if the sysadmin is
messing about (meaning that the timing of callbacks from GTK or
select timeouts is _more_ likely to be right than GETTICKCOUNT).
This checkin provides band-aid workarounds for both problems, which
aren't pretty but ought to at least prevent catastrophic assertion
failure.

git-svn-id: svn://svn.tartarus.org/sgt/putty@5556 cda61777-01e9-0310-a592-d414129be87e