timezone: Correctly resolve symlink from /etc/localtime
The return value of g_file_read_link ("/etc/localtime") can
be a relative path in the form of "../usr/share/zoneinfo".
This breaks the prefix check that is performed, and makes
the timezone identifier be "../usr/share/zoneinfo/America/Sao_Paulo",
for example, which breaks other parts of the system.
Fix that by canonicalizing the symlink path if we detect
is it a relative path.
(Tweaked by Philip Withnall <withnall@endlessm.com> to remove a
conditional which was unnecessary.)
https://bugzilla.gnome.org/show_bug.cgi?id=111848