Hi,

Enclosed is a patch that sets us down the road of understanding EQ/EN 
blocks.  It's quite simple.  Like TBL, it intercepts equations in 
libroff (roff.c).  It then passes back the EQN parse (eqn.c) into the 
main drive (main.c), which (not yet) will push these into libmdoc and 
libman.

The difference between TBL and EQN is that EQN sucks in all stuff 
between .EQ and .EN, while TBL must interpret what happens in these blocks.

For now, the bloat is one more file (the parser for EQN, which is for 
now just a few lines), some conditionals in the libroff parser to check 
for active equation parses, and handling of EQ/EN.

The equations themselves are thrown away.  This is only because I wanted 
to float a patch early with the libroff framework.  It's trivial to put 
an addspan() style routine to both libmdoc and libman that at least 
print out the data.  I'll do that with an Ok or two.

Thoughts?

By the way, we'll be able to fully support "delim", as libroff allows us 
to chop up its input lines and send them to the other backends, so

  foo $a + b$ bar

assuming `$$' are the EQN delimiters, would be passed back as

  foo
  EQN:a + b
  bar

Neat.

Thanks,

Kristaps