Previously, when you selected to have a custom format, whatever was in
the custom-format box would get replaced with the format for
12-hour-without-seconds. Now, it keeps whatever was in the box before -
which is less disorientating, and lets you tweak the existing format.
This means that if you save and close ClockSettings with "24-hour"
and "Show seconds" both checked, then they will both be checked when
you re-open ClockSettings, instead of it showing as a "Custom" format.
This renames the current implementation of current_time_zone to
system_time_zone to more clearly indicate what it is. Then reimplements
current_time_zone to return whatever was set up by tzset, falling back
to UTC if something went awry, for convenience.
