tzcode: Clean up the ctime(3) manual page.

MFC after: 3 weeks
Sponsored by: Klara, Inc.
Reviewed by: pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D39714

(cherry picked from commit 6f3c2f41b18b9a7a8feb3c50254a21db2d9908fd)

libc: Improve description of mktime() / timegm().

• Mention that mktime() and timegm() set errno on failure.
• Correctly determining whether mktime() / timegm() succeeded with arbitrary input (where -1 can be a valid result) is non-trivial. Document the recommended procedure.

PR: 277863
MFC after: 1 week
Reviewed by: pauamma_gundo.com, gbe
Differential Revision: https://reviews.freebsd.org/D44503

(cherry picked from commit 7534109d13a6cdb22e78d9d4c0a0cd5efd323c45)