timer.c: skip intermediate timer intervals

timer.c: skip intermediate timer intervals

"timer_inc()" now checks how many trigger intervals have passed since
a given timer has been updated.  This might be due to "negative clock
skew" (think of summer time) or the fact that some processing took too
long (i.e. fetching of a web site).  These missed trigger intervals
are then skipped and the user is notified so that he may adapt his
timer settings.  This handling is essential, otherwise unprocessed
timers might stack up and would trigger continuously while at the same
time becoming notoriously late and unreliable.

git-svn-id: https://ssl.bulix.org/svn/lcd4linux/trunk@1141 3ae390bd-cb1e-0410-b409-cd5a39f66f1f