SVf_IsCOW

As discussed in ticket #114820, instead of using READONLY+FAKE to mark
a copy-on-write string, we should make it a separate flag.

There are many modules in CPAN (and 1 in core, Compress::Raw::Zlib)
that assume that SvREADONLY means read-only.  Only one CPAN module,
POSIX::pselect will definitely be broken by this.  Others may need to
be tweaked.  But I believe this is for the better.

It causes all tests except ext/Devel-Peek/t/Peek.t (which needs a tiny
tweak still) to pass under PERL_OLD_COPY_ON_WRITE, which is a prereq-
uisite for any new COW scheme that creates COWs under the same cir-
cumstances.

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