Create a GUC variable REGEX_FLAVOR to control the type of regular

Create a GUC variable REGEX_FLAVOR to control the type of regular
expression accepted by the regex operators, per discussion yesterday.

Along the way, reduce deadlock_timeout from PGC_POSTMASTER to PGC_SIGHUP
category.  It is probably best to insist that all backends share the same
setting, but that doesn't mean it has to be frozen at startup.