gtimezone: Fallback to /var/db/zoneinfo on FreeBSD

The timezone setup utility of FreeBSD, tzsetup, which is run during the
installation, creates /etc/localtime by copying the chosen timezone file
from /usr/share/zoneinfo. Although it can correctly deal with the case
where /etc/localtime is a symlink, it is not the default and there is no
user interface to change the default copying behaviour.

Fortunately, tzsetup has been modified to write the name of the chosen
timezone to /var/db/zoneinfo in 2009, so we can know the name of the
current timezone by reading it. DragonflyBSD also seems to do the same
thing in its tzsetup.

https://svnweb.freebsd.org/changeset/base/198267
https://bugzilla.gnome.org/show_bug.cgi?id=795165