FYI: Generate bison.1

[Akim Demaille]

I suppose we should produce some yacc.1, but should it be:

- a stub that says that this is bison -y, read bison(1)

- be a copy of bison.1 with the precision that this is
  bison -y.

- be a copy of bison.1 (with s/bison/yacc/, which is, btw,
  not done in yacc --help).

I think 1 or 2 are ok.

Another question is whether we leave bison.1 in the repository.
I left it there, so that we don't require help2man from maintainers
(it is called via missing).

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