ecpg: major cleanup, simplification, and documentation of parse.pl.

Remove a lot of cruft, clean up and document what's left.
This produces the same preproc.y output as before, except for
fewer blank lines.  (It's not like we're making any attempt to
match the layout of gram.y, so I removed the one bit of logic
that seemed to have that in mind.)

Discussion: https://postgr.es/m/2011420.1713493114@sss.pgh.pa.us

Author: tglsfdc