timedated: gather timezone from /etc/localtime sym target

/etc/localtime -> /usr/share/zoneinfo/...

or

/etc/localtime -> ../usr/share/zoneinfo/...

(note, ../usr is not the same if /etc is a symlink, as this isn't
using canonicalize_file_name())

keep other method for now, consider dropping later.

Supporting relative links here are problematic as timezones in
/usr/share/zoneinfo are often themselves symlinks (and symlinks to
symlinks), so this implamentation only supports absolute symlinks
"/usr/share/zoneinfo/" and relative symlinks starting with
"../usr/share/zoneinfo/"

>From TODO (kay sievers):
* kill /etc/timezone handling entirely? What does it provide?
  - /etc/localtime carries the same information already:
      $ ls -l /etc/localtime; cat /etc/timezone
      lrwxrwxrwx 1 root root 33 Jul 27 09:55 /etc/localtime -> /usr/share/zoneinfo/Europe/Berlin
      Europe/Berlin
  - systemd enforces /usr to be available at bootup, so we can
    enforce the use of the symlink

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