Make sv_set[ps]v(cv...) set prototype

Make sv_set[ps]v(cv...) set prototype

The SvPVX slot of a CV is used both for the prototype and for the sub
name passed to an XS AUTOLOAD sub.

It used to be that such AUTOLOADing would clobber the prototype.

Commit 8fa6a4095 made the two uses of SvPVX try to play along nicely
with each other, so the prototype comes after the sub name if both
need to be present.  It added the CvPROTO macro to account for that.

Some CPAN modules expect to be able to set the prototype with
sv_set[ps]v.  So this commit makes that work as expected, by turn-
ing off the flag that says the prototype comes after the auto-
loaded sub name.

Anyone using Scalar::Util::set_prototype to work around the proto-
type-clobbering bug can now continue to do so, without triggering
a new bug.