[timed] Change location of time zone link that timed manages

Timed links time zone files in /usr/share/zoneinfo to
/var/lib/timed, for example

  /var/lib/timed/localtime -> /usr/share/zoneinfo/Europe/Helsinki

The system time zone /etc/localtime is in turn linked to
/var/lib/timed/localtime, so that it follows the time zone
set by timed.

These changes are required as timed is not run with root
priviledges, and it does not have the rights to change
/etc/localtime.