Remove Giant from settime(), tc_setclock_mtx guards tc_windup() calls,
and there is no other issues with parallel settime(). Remove spl()
vestiges there as well.

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
Differential revision: https://reviews.freebsd.org/D7302