Save and restore PL_regeol for op inside of regex (RT ##66110)

Save and restore PL_regeol for op inside of regex (RT ##66110)

If the op inside of a (?{ }) construct is another regex, the two
regexen end up corrupting each others' end-of-string markers,
resulting in various pathologies including access violations,
stack corruptions, and memory use growing without bound.

The change here is intended to be a relatively safe, cheap way to
prevent memory errors and makes no attempt to save and restore
other aspects of regex state; i.e., general purpose reentrancy
for the regex engine is still a TODO.