Clean up some mess in row-security patches.

Fix unsafe coding around PG_TRY in RelationBuildRowSecurity: can't change
a variable inside PG_TRY and then use it in PG_CATCH without marking it
"volatile".  In this case though it seems saner to avoid that by doing
a single assignment before entering the TRY block.

I started out just intending to fix that, but the more I looked at the
row-security code the more distressed I got.  This patch also fixes
incorrect construction of the RowSecurityPolicy cache entries (there was
not sufficient care taken to copy pass-by-ref data into the cache memory
context) and a whole bunch of sloppiness around the definition and use of
pg_policy.polcmd.  You can't use nulls in that column because initdb will
mark it NOT NULL --- and I see no particular reason why a null entry would
be a good idea anyway, so changing initdb's behavior is not the right
answer.  The internal value of '\0' wouldn't be suitable in a "char" column
either, so after a bit of thought I settled on using '*' to represent ALL.
Chasing those changes down also revealed that somebody wasn't paying
attention to what the underlying values of ACL_UPDATE_CHR etc really were,
and there was a great deal of lackadaiscalness in the catalogs.sgml
documentation for pg_policy and pg_policies too.

This doesn't pretend to be a complete code review for the row-security
stuff, it just fixes the things that were in my face while dealing with
the bugs in RelationBuildRowSecurity.

src/test/regress/expected/rules.out

@@ -1363,16 +1363,13 @@ pg_policies| SELECT n.nspname AS schemaname,
               WHERE (pg_authid.oid = ANY (pol.polroles))
               ORDER BY pg_authid.rolname)
         END AS roles,
-        CASE
-            WHEN (pol.polcmd IS NULL) THEN 'ALL'::text
-            ELSE
-            CASE pol.polcmd
-                WHEN 'r'::"char" THEN 'SELECT'::text
-                WHEN 'a'::"char" THEN 'INSERT'::text
-                WHEN 'u'::"char" THEN 'UPDATE'::text
-                WHEN 'd'::"char" THEN 'DELETE'::text
-                ELSE NULL::text
-            END
+        CASE pol.polcmd
+            WHEN 'r'::"char" THEN 'SELECT'::text
+            WHEN 'a'::"char" THEN 'INSERT'::text
+            WHEN 'w'::"char" THEN 'UPDATE'::text
+            WHEN 'd'::"char" THEN 'DELETE'::text
+            WHEN '*'::"char" THEN 'ALL'::text
+            ELSE NULL::text
         END AS cmd,
     pg_get_expr(pol.polqual, pol.polrelid) AS qual,
     pg_get_expr(pol.polwithcheck, pol.polrelid) AS with_check