Finally! Worked out how to const Perl_moreswitches().

Finally! Worked out how to const Perl_moreswitches().
Although I've committed crimes against indenting to avoid losing
blame annotation history. Revisit this when we have version control
that can easily see through whitespace changes.
[Can probably now propagate consting inside S_parse_body(),
Perl_moreswitches() and Perl_yylex(), but that will not affect the
external API, so can wait until after 5.10]

p4raw-id: //depot/perl@32057