[perl #123103] Allow ext magic on read-onlies

Perl cannot know whether the magic will modify the SV, so it should
give the benefit of the doubt.
[perl #123103] Just set SVf_READONLY on magic vars

a623f8939 was arguably a little too eager.  It’s purpose is to protect
vars whose modification can causes hangs and crashes.  I don’t believe
that is the case for any magic vars.