Make defining both PORTVERSION and DISTVERSION a DEV_ERROR.

There are two cases:

• The upstream versionning is compatible with our versionning, or using DISTVERSION's magic leads to a compatible PORTVERSION, use DISTVERSION. If it is possible to use DISTVERSIONPREFIX and DISTVERSIONSUFFIX to make it compatible, use them.
• The upstream versionning is not compatible with our versionning, and DISTVERSION's magic does not lead to a correct PORTVERSION, then set PORTVERSION to the equivalent of our versionning, and set DISTNAME. It is possible to use a third variable where you store upstream's version and use it to compute PORTVERSION and/or DISTNAME, like the dns/bind9* ports do.

Sponsored by: Absolight