Prevent parallel tc_windup() calls, both parallel top-level calls from
setclock() and from simultaneous top-level and interrupt. For this,
tc_windup() is protected with a tc_setclock_mtx spinlock, in the try
mode when called from hardclock interrupt. If spinlock cannot be
obtained without spinning from the interrupt context, this means that
top-level executes tc_windup() on other core and our try may be
avoided.

The boottimebin and boottime variables should be adjusted from
tc_windup(). To be correct, they must be part of the timehands and
read using lockless protocol. Remove the globals and reimplement the
getboottime(9)/getboottimebin(9) KPI using the timehands read
protocol.

Tested by: pho (as part of the whole patch)
Reviewed by: jhb (same)
Discussed wit: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
X-Differential revision: https://reviews.freebsd.org/D7302