Values.pm: only allow @foo@ subst in same section

This commit fixes @foo@ substitution such that if foo is not defined
in the current section, then @foo@ will evaluate to "" (i.e., an empty
string) rather than a cryptic Perl "HASH(...)" kind of string.
Merge pull request #407 from jsquyres/pr/global-ini-vars

Values.pm: only allow ```@foo@``` substitution within a single section.