tzcode: Implement timezone change detection

Implement optional timezone change detection for local time libc
functions. This is disabled by default; set WITH_DETECT_TZ_CHANGES
to build it.

Reviewed By: imp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #47
Differential Revision: https://reviews.freebsd.org/D30183