Allow regex operations to be terminated early by query cancel requests.

 Allow regex operations to be terminated early by query cancel requests.
 
 The regex code didn't have any provision for query cancel; which is
 unsurprising given its non-Postgres origin, but still problematic since
 some operations can take a long time.  Introduce a callback function to
 check for a pending query cancel or session termination request, and
 call it in a couple of strategic spots where we can make the regex code
 exit with an error indicator.
 
 If we ever actually split out the regex code as a standalone library,
 some additional work will be needed to let the cancel callback function
 be specified externally to the library.  But that's straightforward
 (certainly so by comparison to putting the locale-dependent character
 classification logic on a similar arms-length basis), and there seems
 no need to do it right now.
 
 A bigger issue is that there may be more places than these two where
 we need to check for cancels.  We can always add more checks later,
 now that the infrastructure is in place.
 
 Since there are known examples of not-terribly-long regexes that can
 lock up a backend for a long time, back-patch to all supported branches.
 I have hopes of fixing the known performance problems later, but adding
 query cancel ability seems like a good idea even if they were all fixed.