Clean up includes from RLS patch

The initial patch for RLS mistakenly included headers associated with
the executor and planner bits in rewrite/rowsecurity.h.  Per policy and
general good sense, executor headers should not be included in planner
headers or vice versa.

The include of execnodes.h was a mistaken holdover from previous
versions, while the include of relation.h was used for Relation's
definition, which should have been coming from utils/relcache.h.  This
patch cleans these issues up, adds comments to the RowSecurityPolicy
struct and the RowSecurityConfigType enum, and changes Relation->rsdesc
to Relation->rd_rsdesc to follow Relation field naming convention.

Additionally, utils/rel.h was including rewrite/rowsecurity.h, which
wasn't a great idea since that was pulling in things not really needed
in utils/rel.h (which gets included in quite a few places).  Instead,
use 'struct RowSecurityDesc' for the rd_rsdesc field and add comments
explaining why.

Lastly, add an include into access/nbtree/nbtsort.c for
utils/sortsupport.h, which was evidently missed due to the above mess.

Pointed out by Tom in 16970.1415838651@sss.pgh.pa.us; note that the
concerns regarding a similar situation in the custom-path commit still
need to be addressed.

src/backend/rewrite/rowsecurity.c

@@ -300,7 +300,7 @@ pull_row_security_policies(CmdType cmd, Relation relation, Oid user_id)
 	 * There must always be at least one policy defined (may be the simple
 	 * 'default-deny' policy, if none are explicitly defined on the table).
 	 */
-	foreach(item, relation->rsdesc->policies)
+	foreach(item, relation->rd_rsdesc->policies)
 	{
 		policy = (RowSecurityPolicy *) lfirst(item);