Remove some excess cleverness from the Bhk macros.

Remove some excess cleverness from the Bhk macros.

Allowing BhkENTRY(bhk, start) to look up the bhk_start member defeats
much of the point of having a bhk_ prefix in the first place: if a
member is added later called (say) 'bhk_die', any invocation of
BhkENTRY(bhk, die) will expand to BhkENTRY(bhk, Perl_die) because of the
API macros. Requiring BhkENTRY(bhk, bhk_start), while tedious, is much
safer.