Renumber SVf_IsCOW

and free up a bit.
Peek.t fix-up
Add SVf_PROTECT
Implement the bipolar read-only system

This fixes bugs related to Hash::Util::unlock accidentally unlocking
internal scalars (e.g., that returned by undef()) and allowing them to
be modified.

Internal read-only values are now marked by two flags, the regular
read-only flag, and the new ‘protected’ flag.

Before this SvREADONLY served two purposes:

1) The code would use it to protect things that must not be modi-
   fied, ever (except when the core sees fit to do so).
2) Hash::Util and everybody else would use it to make this unmodifia-
   ble temporarily when requested by the user.

Internals::SvREADONLY serves the latter purpose and only flips the
read-only flag, so things that need to stay read-only will remain so,
because of the ‘other’ read-only flag, that CPAN doesn’t know about.
(If you are a CPAN author, do not read this.)
More Peek.t fix-ups
[perl #115254] Fix flag check on scope exit

$ ./perl -Ilib -e '{ my $x = 3; Internals::SvREADONLY $x, 1; () }'
$ ./perl -Ilib -e '{ my $x = ${qr//}; Internals::SvREADONLY $x, 1; () }'
Modification of a read-only value attempted at -e line 1.

The latter causes $x to be marked FAKE.  At the time this code was
introduced in scope.c, read-only+fake meant cow, so the !fake check
was necessary.  (That said, it has always behaved incorrectly for glob
copies that are also marked fake.)
Fix read-only flag checks in lvalue sub exit

See the previous commit for the explanation.  This fixes this
discrepancy:

$ ./miniperl -Ilib -e '+sub:lvalue{my $x = 3; Internals::SvREADONLY $x, 1; $x }->() = 3'
Can't return a readonly value from lvalue subroutine at -e line 1.
$ ./miniperl -Ilib -e '+sub:lvalue{my $x = *foo; Internals::SvREADONLY $x, 1; $x }->() = 3'
Modification of a read-only value attempted at -e line 1.

It was not just a flag check that this commit fixed, but also a bogus
SvREADONLY(TOPs) where TOPs may not even be the scalar we are dying
for, giving ‘a temporary’ for some read-only values.  That mistake was
my own, made in commit d25b0d7b8.