Framework: Allow setting the 'vital' flag from a port

[arrowd]

Summary

Rationale for this change is following.

In our project we have a large meta-port that represents all the 3rd-party
software that makes up our distribution. This is sort of pkgbase, but at the
higher level.

We want to make sure this metapackage and all its dependents are never removed,
hence need to set the vital flag from the port.

This feature is not intended to be used in the main branch, but may be useful
for enterprise consumers of the Ports tree. Maybe the knob should be even named
_VITAL, bikeshedding is welcome.

Test Plan

make create-manifest contains vital : true for a port containing VITAL=yes

[bapt]

We should probably also add a comment in bsd.port.mk which clearly state this should not be used directly in the ports tree, but that is another story