harmonise two versions of Perl_my_cxt_init

harmonise two versions of Perl_my_cxt_init

There are two separate copies of this function, chosen by
the presence of PERL_GLOBAL_STRUCT_PRIVATE.

Make the two versions closer to each other, to allow shortly for
merging.

Mainly its renaming the int pointer parameter from 'index' to 'indexp',
and adding a local var 'index', which is assigned from *indexp where
appropriate.

Also harmonise some of the comments between the two functions.