mktables: Generalize the scx property handling

mktables: Generalize the scx property handling

Until now, this property was unique in that it specifies a set of
possible values for scripts that a character can be in, rather than a
single script.  That multiplicity has been handled specially.  But the
next couple of commits will introduce another property that has similar
characteristics.  This commit makes the scx handling more general, so as
to also be usable for the new property.