Add an Emacs header line to output files that makes generated .c files

Add an Emacs header line to output files that makes generated .c files
read-only by default, to make it difficult to accidentally change
generated files.

FREQUENCIES: Compare by frequency, not bogus a->v.c <=> b->v.c pointer
compare.

Changed data_out() to store string data directly into a `union
value''s s member, not indirectly into c.

Renamed some confusingly named member objects in crosstabs.q.