pgAdmin3: change variant names

[ryandesign (Ryan Carsten Schmidt)]

$ port info pgAdmin3 |head -n 2
pgAdmin3 @1.16.0 (aqua)
Variants:             debug, with_postgresql83, with_postgresql84, with_postgresql90, with_postgresql91, [+]with_postgresql92

The variant names should be changed. Variant names should not begin with a "with_" prefix.

The old variant names should be retained for one year, marked as requiring the new variant names, to facilitate upgrades.

The attached patch does this, and simplifies the variants as well by removing unnecessary directives.

The way the default variant is selected is also improved so that users wishing to use a non-default variant do not encounter an error message about variant conflicts.

[jyrkiwahlstedt]

done in r98257