[timed] Do not link to time zone information in post install

Time zone may be set during image creation by copying a file
from /usr/share/zoneinfo to /etc/localtime. If the link chain
/etc/localtime -> /var/lib/timed/localtime -> /usr/share/zoneinfo/<file>
exists, then <file> in /usr/share/zoneinfo/<file> will be
overwritten.

Set the UTC time zone as the default timed time zone by
copying /usr/share/zoneinfo/UTC to /var/lib/timed/localtime
during post install. Overwriting /etc/localtime will then
not overwrite files in /usr/share/zoneinfo.

Timed changes the file in /var/lib/timed/localtime into a link
while setting the time zone during runtime, see
source_settings::fix_etc_localtime().
[timed] Do not remove time zone link if target does not exist

If the time zone that timed is trying to set does not exist
in directory /usr/share/zoneinfo, then do not remove the existing
link /var/lib/timed/localtime -> usr/share/zoneinfo/<file>.
Merge pull request #25 from pgerdt/fix-timezone-linking

Fix bugs in setting the time zone