Recognize timezones on FreeBSD for the default install.

[jhb]

Summary

By default, FreeBSD copies the current timezone database to /etc/localtime
and stores the name of the symlink in /var/db/zoneinfo. Qt expects
/etc/localtime to be a symlink and uses the symlink's target to infer the
name of the default timezone. The existing code in Qt includes some
workarounds for Linux distributions that also copy the current timezone to
/etc/localtime by checking for files that contain the name of the timezone.
Add a similar check for /var/db/zoneinfo on FreeBSD.

PR: 200763

Test Plan

• this fixes the clock in the panel to correctly honor the default timezone when /etc/localtime is a regular file

[tcberner]

I had a similar patch prepared but seem to have forgotten about it :)