Improve the readability of the produced parsers.

* src/bison.s1: Formatting changes.
Improve the comment related to the `$' mark.
(yydefault): Don't fall through to `yyresume': `goto' there.
* src/output.c (output_parser): When the `$' is met, skip the end
of its line.
New variable, `number_of_dollar_signs', to check there's exactly
one `$' in the parser skeleton.