Cleanup of rewriter and planner handling of Query.hasRowSecurity flag.

Be sure to pull up the subquery's hasRowSecurity flag when flattening a
subquery in pull_up_simple_subquery().  This isn't a bug today because
we don't look at the hasRowSecurity flag during planning, but it could
easily be a bug tomorrow.

Likewise, make rewriteRuleAction() pull up the hasRowSecurity flag when
absorbing RTEs from a rule action.  This isn't a bug either, for the
opposite reason: the flag should never be set yet.  But again, it seems
like good future proofing.

Add a comment explaining why rewriteTargetView() should *not* set
hasRowSecurity when adding stuff to securityQuals.

Improve some nearby comments about securityQuals processing, and document
that field more completely in parsenodes.h.

Patch by me, analysis by Dean Rasheed.

Discussion: <CAEZATCXZ8tb2DV6f=bkhsMV6u_gRcZ0CZBw2J-qU84RxSukZog@mail.gmail.com>

src/include/nodes/parsenodes.h

@@ -775,6 +775,13 @@ typedef struct XmlSerialize
  *	  FirstLowInvalidHeapAttributeNumber from column numbers before storing
  *	  them in these fields.  A whole-row Var reference is represented by
  *	  setting the bit for InvalidAttrNumber.
+ *
+ *	  securityQuals is a list of security barrier quals (boolean expressions),
+ *	  to be tested in the listed order before returning a row from the
+ *	  relation.  It is always NIL in parser output.  Entries are added by the
+ *	  rewriter to implement security-barrier views and/or row-level security.
+ *	  Note that the planner turns each boolean expression into an implicitly
+ *	  AND'ed sublist, as is its usual habit with qualification expressions.
  *--------------------
  */
 typedef enum RTEKind
@@ -872,7 +879,7 @@ typedef struct RangeTblEntry
 	Bitmapset  *selectedCols;	/* columns needing SELECT permission */
 	Bitmapset  *insertedCols;	/* columns needing INSERT permission */
 	Bitmapset  *updatedCols;	/* columns needing UPDATE permission */
-	List	   *securityQuals;	/* any security barrier quals to apply */
+	List	   *securityQuals;	/* security barrier quals to apply, if any */
 } RangeTblEntry;
 
 /*