Serialize tc_windup() calls and modification of some timehands members.
If a user thread from e.g. clock_settime(2) is in the midst of changing
the boottime or calling tc_windup() when it is interrupted by hardclock(9),
the timehands could be left in a damaged state.
So protect tc_windup() calls with a mutex, timecounter_mtx. hardclock(9)
merely attempts to enter the mutex instead of spinning because it cannot
afford to wait around. In practice hardclock(9) will skip tc_windup() very
rarely, and when it does skip there aren't any negative effects because the
skip indicates that a user thread is already calling, or about to call,
tc_windup() anyway.
Based on FreeBSD r303387 and NetBSD sys/kern/kern_tc.c,v1.30
Discussed with mpi@ and visa@. Tons of nice technical detail about
lockless reads from visa@.
OK visa@
UnifiedSplitRaw
