basic: add string table macros for "extended boolean" enums

basic: add string table macros for "extended boolean" enums

In a couple of cases we maintain configuration settings that know an on
and off state, like a boolean, plus some additional states. We generally
parse them as booleans first, and if that fails check for specific
additional values.

This adds a generalized set of macros for parsing such settings, and
ports one use in resolved and another in networkd over to it.