Complete bsd.cov.mk integration

Pass through appropriate objects via COV_OBJS in bsd.lib.mk (based on
SOBJS), and appropriate objects via bsd.prog.mk (OBJS).

While here, do more intelligent things in terms of iterating over COV_OBJS,
building lists of items to install with the GCNOS variable/adding GCNOS to
FILESGROUPS, etc. This reduces the 2 unrolled .for loops into 1.