* NEWS: Document when yyparse started to return 2.

* doc/bison.texinfo (Parser Function): Document when yyparse
returns 2.

* data/lalr1.cc: Revert part of previous change, as it's incompatible.
(b4_filename_type): Renamed back from b4_file_name_type.  All uses
changed.
(class position): file_name -> filename (reverting).  All uses changed.