RLS refactoring

This refactors rewrite/rowsecurity.c to simplify the handling of the
default deny case (reducing the number of places where we check for and
add the default deny policy from three to one) by splitting up the
retrival of the policies from the application of them.

This also allowed us to do away with the policy_id field.  A policy_name
field was added for WithCheckOption policies and is used in error
reporting, when available.

Patch by Dean Rasheed, with various mostly cosmetic changes by me.

Back-patch to 9.5 where RLS was introduced to avoid unnecessary
differences, since we're still in alpha, per discussion with Robert.

src/include/nodes/parsenodes.h

@@ -928,6 +928,7 @@ typedef struct WithCheckOption
 	NodeTag		type;
 	WCOKind		kind;			/* kind of WCO */
 	char	   *relname;		/* name of relation that specified the WCO */
+	char	   *polname;		/* name of RLS policy being checked */
 	Node	   *qual;			/* constraint qual to check */
 	bool		cascaded;		/* true for a cascaded WCO on a view */
 } WithCheckOption;