Declare subjects instead of picking them up automatically

Declare subjects instead of picking them up automatically

Since subjects were added in Empire 2, we've always picked them up
from .SA requests.  If you mistype a subject there, you get a "is a
NEW subject" warning, and incorrect subject pages.  When building a
pristine tree, you get bogus "is a NEW subject" warnings for all
subjects.  If you somehow delete the generated subjects.mk, but not
the generated subject files, the build breaks.

Declare subjects in Make variable subjects.  Drop generated makefile
subject.mk.

Treat unknown topics in .SA arguments as errors.  This replaces the
"$subj is a NEW subject" warning.

Treat subjects without member pages as errors.  This replaces the "The
subject $subj has been removed" warning.

Safer and simpler.