Allow downstream projects to easily add private and internal libs

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.

Reviewed by: bdrewery
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D33901