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/storage/buffer/localbuf.c

@@ -361,7 +361,7 @@ DropRelFileNodeLocalBuffers(RelFileNode rnode, ForkNumber forkNum,
  *		This function removes from the buffer pool all pages of all forks
  *		of the specified relation.
  *
- *		See DropRelFileNodeAllBuffers in bufmgr.c for more notes.
+ *		See DropRelFileNodesAllBuffers in bufmgr.c for more notes.
  */
 void
 DropRelFileNodeAllLocalBuffers(RelFileNode rnode)

src/backend/storage/ipc/procarray.c

@@ -1428,10 +1428,11 @@ GetOldestXmin(Relation rel, int flags)
 		result = replication_slot_xmin;
 
 	/*
-	 * After locks have been released and defer_cleanup_age has been applied,
-	 * check whether we need to back up further to make logical decoding
-	 * possible. We need to do so if we're computing the global limit (rel =
-	 * NULL) or if the passed relation is a catalog relation of some kind.
+	 * After locks have been released and vacuum_defer_cleanup_age has been
+	 * applied, check whether we need to back up further to make logical
+	 * decoding possible. We need to do so if we're computing the global limit
+	 * (rel = NULL) or if the passed relation is a catalog relation of some
+	 * kind.
 	 */
 	if (!(flags & PROCARRAY_SLOTS_XMIN) &&
 		(rel == NULL ||

src/backend/storage/lmgr/predicate.c

@@ -4360,7 +4360,7 @@ CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
 	/*
 	 * If we found one of our own SIREAD locks to remove, remove it now.
 	 *
-	 * At this point our transaction already has an ExclusiveRowLock on the
+	 * At this point our transaction already has a RowExclusiveLock on the
 	 * relation, so we are OK to drop the predicate lock on the tuple, if
 	 * found, without fearing that another write against the tuple will occur
 	 * before the MVCC information makes it to the buffer.

src/backend/storage/smgr/md.c

@@ -1163,9 +1163,6 @@ _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
 			 * replaying WAL data that has a write into a high-numbered
 			 * segment of a relation that was later deleted. We want to go
 			 * ahead and create the segments so we can finish out the replay.
-			 * However if the caller has specified
-			 * EXTENSION_REALLY_RETURN_NULL, then extension is not desired
-			 * even in recovery; we won't reach this point in that case.
 			 *
 			 * We have to maintain the invariant that segments before the last
 			 * active segment are of size RELSEG_SIZE; therefore, if