Consistently use "superuser" instead of "super user"

The correct nomenclature for the highest privileged user is superuser
and not "super user", this replaces the few instances where that was
used erroneously. No user-visible changes are done as all changes are
in comments, so no back-patching.

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CALj2ACW3snGBD8BAQiArMDS1Y43LuX3ymwO+N8aUg1Hrv6hYNw@mail.gmail.com

src/backend/commands/amcmds.c

@@ -53,7 +53,7 @@ CreateAccessMethod(CreateAmStmt *stmt)
 
 	rel = table_open(AccessMethodRelationId, RowExclusiveLock);
 
-	/* Must be super user */
+	/* Must be superuser */
 	if (!superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

src/backend/commands/foreigncmds.c

@@ -573,7 +573,7 @@ CreateForeignDataWrapper(ParseState *pstate, CreateFdwStmt *stmt)
 
 	rel = table_open(ForeignDataWrapperRelationId, RowExclusiveLock);
 
-	/* Must be super user */
+	/* Must be superuser */
 	if (!superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
@@ -690,7 +690,7 @@ AlterForeignDataWrapper(ParseState *pstate, AlterFdwStmt *stmt)
 
 	rel = table_open(ForeignDataWrapperRelationId, RowExclusiveLock);
 
-	/* Must be super user */
+	/* Must be superuser */
 	if (!superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

src/backend/commands/tablespace.c

@@ -242,7 +242,7 @@ CreateTableSpace(CreateTableSpaceStmt *stmt)
 	Oid			ownerId;
 	Datum		newOptions;
 
-	/* Must be super user */
+	/* Must be superuser */
 	if (!superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

src/test/regress/expected/conversion.out

@@ -33,7 +33,7 @@ DROP CONVERSION mydef;
 -- so there's no need to do that here.
 --
 --
--- return to the super user
+-- return to the superuser
 --
 RESET SESSION AUTHORIZATION;
 DROP USER regress_conversion_user;

src/test/regress/sql/conversion.sql

@@ -30,7 +30,7 @@ DROP CONVERSION mydef;
 -- so there's no need to do that here.
 --
 --
--- return to the super user
+-- return to the superuser
 --
 RESET SESSION AUTHORIZATION;
 DROP USER regress_conversion_user;