gv.c: gv_name_set and gv_init_(etc) now initialize the GV's name as UTF-8 if passed...

gv.c: gv_name_set and gv_init_(etc) now initialize the GV's name as UTF-8 if passed the UTF8 flag.

newCONSTSUB is still unclean however, so constant subs are
still generated under a wrong name.

gv_fullname4 is also UTF-8 aware now; While that should've gotten
it's own commit and tests, it's not possible to test the
UTF-8 part without the gv_init changes, and it's not possible
to test the gv_init changes without gv_fullname4.
Chicken and egg, as it were. So let's compromise and
wait for the relevant tests once globs can be intiialized as
UTF-8 from the Perl level without XS magic.