Add check_keyword.pl script to perform some basic sanity checks to the

Add check_keyword.pl script to perform some basic sanity checks to the
keyword lists in gram.y and kwlist.h. It checks that all lists are in
alphabetical order, and that all keywords present in gram.y are listed
in kwlist.h in the right category, and that all keywords in kwlist.h are
also in gram.y. What's still missing is to check that all keywords
defined  with "%token <keyword>" in gram.y are present in one of the
keyword lists in gram.y.