libc: Add "Z" as TZ designator for strptime.

ISO 8601 allows use of "Z" as the time zone designator. Update the
strptime parser to allow this usage.

While we are at it, update the manpage to reflect that both UTC and Z
are now valid options.

Reviewed by: des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53083