MFC r303382:
Provide the getboottime(9) and getboottimebin(9) KPI.

MFC r303387:
Prevent parallel tc_windup() calls. Keep boottime in timehands,
and adjust it from tc_windup().

MFC notes:

The boottime and boottimebin globals are still exported from
the kernel dyn symbol table in stable/11, but their declarations are
removed from sys/time.h. This preserves KBI but not KPI, while all
in-tree consumers are converted to getboottime().

The variables are updated after tc_setclock_mtx is dropped, which gives
approximately same unlocked bugs as before.

The boottime and boottimebin locals in several sys/kern_tc.c functions
were renamed by adding the '_x' suffix to avoid name conficts.