syslogd: re-read the timezone when receive a SIGHUP.

syslogd already re-reads the configuration file and the hostname when
receiving a SIGHUP, so it makes sense to reset the timezone.
Special care was taken to ensure we don't break installations running
with a custom TZ variable.

PR: 196905
Submitted by: Kurt Lidl <lidl pix.net>
Sponsored by: Pi-Coral, Inc.