timeout(9): remove TIMEOUT_KCLOCK flag

timeout(9): remove TIMEOUT_KCLOCK flag

I never should have added the TIMEOUT_KCLOCK flag.  It is redundant
and only serves to complicate the timeout(9) logic.  In every place
where we check for the flag we can just use timeout.to_kclock.

So, remove the flag from <sys/timeout.h> and rewrite all affected
logic to use the value of timeout.to_kclock instead.

ok kn@