Rework privatelib/internallib

Make sure everything linking to a privatelib and/or an internallib does it directly
from the OBJDIR rather than DESTDIR.
Add src.libnames.mk so bsd.libnames.mk is not polluted by libraries not existsing
in final installation
Introduce the LD* variable which is what ld(1) is expecting (via LDADD) to link to
internal/privatelib
Directly link to the .so in case of private library to avoid having to complexify
LDFLAGS.

Phabric: https://phabric.freebsd.org/D553
Reviewed by: imp, emaste