This merges the 'clock_timezone_type' and 'clock_custom_timezone' variables, since the values are exclusive to each other.
It makes the surrounding a little (!) heavier, but I think it is cleaner that way. Also comes with a little code clean-up, for instance the help text for D7 was still referencing D6 paths, and the SimpleTests had all the t()'s.
Haven't written upgrade path yet (For D6).
Note that this doesn't pass the tests yet, will have to clean it up before committing.