Add explicit_bzero(3) and its kernel counterpart.

Obtained from:	OpenBSD
MFC after:	2 weeks
Bump __FreeBSD_version for the addition of explicit_bzero(3).
Add MLINK for explicit_bzero(3) and bump .Dd date.

MFC after:	2 weeks
Back out timegm error check from r272562.

POSIX treats negative time_t as undefined (i.e. may be valid too,
depends on system's policy we don't have) and we don't set EOVERFLOW
in mktime/timegm as POSIX requires to surely distinguish -1 return
as valid negative time from -1 as error return.
1) Fix the case we have less arguments for format string than we expected.
2) Return error on unsupported format specs.
(both according to POSIX)

PR:     93197
Merge remote-tracking branch 'freebsd/master' into hardened/current/master