re-enable Copy-on-Write by default.

COW was first introduced (and enabled by default) in 5.17.7.
It was disabled by default in 5.17.10, because it was though to have too
many rough edges for the 5.18.0 release.

By re-enabling it now, early in the 5.19.x release cycle, hopefully it
will be ready for production use by 5.20.

This commit mainly reverts 9f351b45f4 and e1fd41328c (with modifications),
then updates perldelta.
perl -V displayed PERL_NEW_COPY_ON_WRITE twice

I didn't realise that the perl -V list of defines is now split between
perl.h and perl.c and so added it to one (thinking it was missing) when it
was already in the other).