Allow downstream projects to easily add private and internal libs

[vangyzen]

Summary

Allow projects based on the FreeBSD tree to append to _PRIVATELIBS
and _INTERNALLIBS by simply maintaining their own lists of
LOCAL_PRIVATELIBS and LOCAL_INTERNALLIBS, respectively.

MFC after: 1 week
Sponsored by: Dell EMC Isilon

Test Plan

This allowed a downstream project to build internal libraries with PIE.

[emaste]

This seems reasonable to me, but I think we should have some documentation. Ideally we'd have a man page that documents these sorts of things, but at least a comment in here I'd say.

[vangyzen]

• CR: add comments

[vangyzen]

In D33901#766597, @emaste wrote:

This seems reasonable to me, but I think we should have some documentation. Ideally we'd have a man page that documents these sorts of things, but at least a comment in here I'd say.

Agreed. This is the best I could do. Feel free to suggest something more helpful.