Change the way we handle multiple lex scanners and yacc parsers

Change the way we handle multiple lex scanners and yacc parsers
in a single program. Our old patch to ylwrap does not work anymore
because the bison parsers need shared symbols that start with yy
and we can't easily select which ones work. So now we use -P and
-p to let lex and yacc do the work for us. This requires a patch
to ylwrap, but it is smaller.