Fix inconsistencies in the code

This addresses a couple of issues in the code:
- Typos and inconsistencies in comments and function declarations.
- Removal of unreferenced function declarations.
- Removal of unnecessary compile flags.
- A cleanup error in regressplans.sh.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/0c991fdf-2670-1997-c027-772a420c4604@gmail.com

src/backend/replication/basebackup.c

@@ -1074,7 +1074,7 @@ sendDir(const char *path, int basepathlen, bool sizeonly, List *tablespaces,
 		 * error in that case. The error handler further up will call
 		 * do_pg_abort_backup() for us. Also check that if the backup was
 		 * started while still in recovery, the server wasn't promoted.
-		 * dp_pg_stop_backup() will check that too, but it's better to stop
+		 * do_pg_stop_backup() will check that too, but it's better to stop
 		 * the backup early than continue to the end and fail there.
 		 */
 		CHECK_FOR_INTERRUPTS();