make the version number stuff mandatory.

make the version number stuff mandatory.

Restrict specifications to the parts that actually make sense (in particular,
remove most wildcard support, it's no longer a good idea in true
version number handling), explain about v*, and be more specific about
default package specs (since the fuzzyness in the original description
predates several changes to bsd.port.mk).

Also add version numbers and wildcards where they make sense so the examples
are more complete.