* tzselect.ksh: Compute list of continents from the time zone table,

so that 'tzselect -t time' doesn't create a blind alley for the Arctic.
tzselect: -c COORD now supports more ISO 6709 forms

* tzselect.ksh (LC_ALL): Don't set this, so that the user can
use the locale's decimal point in coordinates.
(convert_coord): Rename from cvt1.  All callers changed.
Support more ISO 6709 forms.
* tzselect.8: Document -c better, including the new forms.