Merge perl_keyword.pl into regen/keywords.pl, to generate keywords.[ch]

Merge perl_keyword.pl into regen/keywords.pl, to generate keywords.[ch]

Prepend + or - in front of all the keyword names in __DATA__ to mark weak and
strong keywords, needed for keyword.c

As keywords.c needs Devel::Tokenizer::C 0.05, not a core module (and not a
common module either) we can no longer run it as part of regen.pl. So store
the sha256 of the source in the generated files, and use this in to check that
they are not stale (in t/porting/regen.t)