basic: validate timezones in get_timezones()

basic: validate timezones in get_timezones()

Depending on the packaging of tzdata, /usr/share/zoneinfo/tzdata.zi may
reference zones or links that are not actually present on the system.
E.g. on Debian and Ubuntu, there is a tzdata-legacy package that
contains "legacy" zones and links, but they are still referenced in
/usr/share/zoneinfo/tzdata.zi shipped by the main tzdata package.

Right now, get_timezoes() does not validate timezones when building the
list, which makes the following possible:

 $ timedatectl list-timezones | grep "US/Alaska"
 US/Alaska

 $ timedatectl set-timezone US/Alaska
 Failed to set time zone: Invalid or not installed time zone 'US/Alaska'

which feels buggy. Hence, simply validate timezones in get_timezones()
to avoid listing timezones that are not installed.