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.

src/backend/utils/misc/postgresql.conf.sample

@@ -208,6 +208,7 @@
 #max_expr_depth = 10000		# min 10
 #max_files_per_process = 1000	# min 25
 #password_encryption = true
+#regex_flavor = advanced	# advanced, extended, or basic
 #sql_inheritance = true
 #transform_null_equals = false
 #statement_timeout = 0		# 0 is disabled, in milliseconds