Use time_t for intermediate values to avoid overflow in clock_ts_to_ct

Use time_t for intermediate values to avoid overflow in clock_ts_to_ct

Add additionally safety and overflow checks to clock_ts_to_ct and the
BCD routines while we're here.

Perform a safety check in sys_clock_settime() first to avoid easy local
root panic, without having to propagate an error value back through
dozens of APIs currently lacking error returns.

PR:		211960, 214300
Submitted by:	Justin McOmie <justin.mcomie at gmail.com>, kib@
Reported by:	Tim Newsham <tim.newsham at nccgroup.trust>
Reviewed by:	kib@
Sponsored by:	Dell EMC Isilon, FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D9279