Disable PL_sawampersand

Disable PL_sawampersand

PL_sawampersand actually causes bugs (e.g., perl #4289), because the
behaviour changes.  eval '$&' after a match will produce different
results depending on whether $& was seen before the match.

Using copy-on-write for the pre-match copy (preceding patches do that)
alleviates the slowdown caused by mentioning $&.  The copy doesn’t
happen unless the string is modified after the match.  It’s now a
post- match copy.  So we no longer need to do things differently
depending on whether $& has been seen.

PL_sawampersand is now #defined to be equal to what it would be if
every program began with $',$&,$`.

I left the PL_sawampersand code in place, in case this commit proves
immature.  Running Configure with -Accflags=PERL_SAWAMPERSAND will
reënable the PL_sawampersand mechanism.