Values.pm: allow for global @vars@ in INI files

This commit introduces new functionality: the ability to read a value
from a different section.  For example, if you have:

```ini
[MTT]
foo = bar

[some_other_section]
yow = @MTT:foo@
```

Then yow will properly get the value of "bar".

Prior to this commit, we already had the @foo@ kind of substitution,
but if the name "foo" was not defined in the current section, the
results were ill-defined, and you'll likely get a cryptic Perl
"HASH(...)" kind of string back.

This commit adds the capability of specifying a section from which to
get the value from -- the format is ```@section:name@```.

This commit also fixes @foo@ substitution such that if foo is not
defined in the current section, @foo@ will evaluate to "" (i.e., an
empty string) rather than a cryptic Perl "HASH(...)" kind of string.