Integrate superuser check into has_rolreplication()

This makes it consistent with similar functions like
has_createrole_privilege() and allows removing some explicit superuser
checks.

Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/20230310000313.GA3992372%40nathanxps13

src/backend/replication/slot.c

@@ -1140,7 +1140,7 @@ CheckSlotRequirements(void)
 void
 CheckSlotPermissions(void)
 {
-	if (!superuser() && !has_rolreplication(GetUserId()))
+	if (!has_rolreplication(GetUserId()))
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 				 errmsg("must be superuser or replication role to use replication slots")));

src/backend/utils/init/miscinit.c

@@ -709,6 +709,10 @@ has_rolreplication(Oid roleid)
 	bool		result = false;
 	HeapTuple	utup;
 
+	/* Superusers bypass all permission checking. */
+	if (superuser_arg(roleid))
+		return true;
+
 	utup = SearchSysCache1(AUTHOID, ObjectIdGetDatum(roleid));
 	if (HeapTupleIsValid(utup))
 	{

src/backend/utils/init/postinit.c

@@ -962,7 +962,7 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	{
 		Assert(!bootstrap);
 
-		if (!superuser() && !has_rolreplication(GetUserId()))
+		if (!has_rolreplication(GetUserId()))
 			ereport(FATAL,
 					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 					 errmsg("must be superuser or replication role to start walsender")));