Rename pg_rowsecurity -> pg_policy and other fixes

As pointed out by Robert, we should really have named pg_rowsecurity
pg_policy, as the objects stored in that catalog are policies.  This
patch fixes that and updates the column names to start with 'pol' to
match the new catalog name.

The security consideration for COPY with row level security, also
pointed out by Robert, has also been addressed by remembering and
re-checking the OID of the relation initially referenced during COPY
processing, to make sure it hasn't changed under us by the time we
finish planning out the query which has been built.

Robert and Alvaro also commented on missing OCLASS and OBJECT entries
for POLICY (formerly ROWSECURITY or POLICY, depending) in various
places.  This patch fixes that too, which also happens to add the
ability to COMMENT on policies.

In passing, attempt to improve the consistency of messages, comments,
and documentation as well.  This removes various incarnations of
'row-security', 'row-level security', 'Row-security', etc, in favor
of 'policy', 'row level security' or 'row_security' as appropriate.

Happy Thanksgiving!

src/include/rewrite/rowsecurity.h

@@ -19,7 +19,7 @@
 
 typedef struct RowSecurityPolicy
 {
-	Oid					rsecid;			/* OID of the policy */
+	Oid					policy_id;		/* OID of the policy */
 	char			   *policy_name;	/* Name of the policy */
 	char				cmd;			/* Type of command policy is for */
 	ArrayType		   *roles;			/* Array of roles policy is for */
@@ -30,8 +30,8 @@ typedef struct RowSecurityPolicy
 
 typedef struct RowSecurityDesc
 {
-	MemoryContext		rscxt;		/* row-security memory context */
-	List			   *policies;	/* list of row-security policies */
+	MemoryContext		rscxt;		/* row security memory context */
+	List			   *policies;	/* list of row security policies */
 } RowSecurityDesc;
 
 /* GUC variable */