gv.c: Begin splitting gv_fetchpvn_flags into smaller helper functions.

This commit takes a chunk of code out of gv_fetchpvn_flags and
turns it into two fuctions: parse_gv_stash_name and find_default_stash.
gv.c: Move the code that magicalizes new globs into magicalize_gv().
gv_fetchpvn_flags: Simplify some warnings code

This line originally checked isLEXWARN_on and had two different code
paths depending on that -- one of which called ckWARN(), which already
checks for isLEXWARN_off, so rather than doing duplicate work, we
just call ckWARN().
gv.c: Explain the purpose of the <none>:: stash a bit.

It's primarily a historical oddity. Its main purpose is to store
variables after an exception from 'use strict "vars"':

    eval q{ use strict; $foo = 1 };
    warn keys %{"<none>::"};
gv.c, gv_fetchpvn_flags: Comments for when the glob already exists.
gv.c, gv_fetchpvn_flags: Split another chunk of magic-checking code.

This bit is called when a GV already exists, but it's name is length-one
and it's on the main:: stash, so it might have multiple kinds of magic,
like $! and %!, or @+ and %+.
gv.c: Rename magicalize_gv into gv_magicalize, make it more specific.

Namely, gv_magicalize no longer stores the GV into the stash, which
is gv_fetchpvn_flags' job.
gv.c: Split part of find_default_stash into gv_is_in_main.

gv_is_in_main() checks if an unqualified identifier is in the main::
stash.
[perl #118091] Split gv_fetchpvn_flags into smaller functions