In retrospect I'm surprised this wasn't thought of before.  With this
you can for example enable backreferences without having to turn on
extendedglob for the whole surrounding context.

A different character could be chosen if (*) seems unwise for some
reason.  It should also be fairly straightforward to make (**) toggle
the option if there's a reason to force extendedglob off.

Patch includes (very brief) doc, and a test case.