Fix misc typos, mostly in comments.

A collection of typos I happened to spot while reading code, as well as
grepping for common mistakes.

Backpatch to all supported versions, as applicable, to avoid conflicts
when backporting other commits in the future.

contrib/pg_upgrade/tablespace.c

@@ -36,7 +36,7 @@ init_tablespaces(void)
  * get_tablespace_paths()
  *
  * Scans pg_tablespace and returns a malloc'ed array of all tablespace
- * paths. Its the caller's responsibility to free the array.
+ * paths. It's the caller's responsibility to free the array.
  */
 static void
 get_tablespace_paths(void)

src/backend/commands/cluster.c

@@ -1518,7 +1518,7 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap,
 	 * swap_relation_files()), thus relfrozenxid was not updated. That's
 	 * annoying because a potential reason for doing a VACUUM FULL is a
 	 * imminent or actual anti-wraparound shutdown.  So, now that we can
-	 * access the new relation using it's indices, update relfrozenxid.
+	 * access the new relation using its indices, update relfrozenxid.
 	 * pg_class doesn't have a toast relation, so we don't need to update the
 	 * corresponding toast relation. Not that there's little point moving all
 	 * relfrozenxid updates here since swap_relation_files() needs to write to

src/interfaces/ecpg/ecpglib/execute.c

@@ -1304,8 +1304,8 @@ ecpg_build_params(struct statement * stmt)
 		if ((position = next_insert(stmt->command, position, stmt->questionmarks, std_strings) + 1) == 0)
 		{
 			/*
-			 * We have an argument but we dont have the matched up placeholder
-			 * in the string
+			 * We have an argument but we don't have the matched up
+			 * placeholder in the string
 			 */
 			ecpg_raise(stmt->lineno, ECPG_TOO_MANY_ARGUMENTS,
 					   ECPG_SQLSTATE_USING_CLAUSE_DOES_NOT_MATCH_PARAMETERS,

src/pl/tcl/pltcl.c

@@ -2337,7 +2337,7 @@ pltcl_SPI_execute_plan(ClientData cdata, Tcl_Interp *interp,
 	}
 
 	/************************************************************
-	 * If there was a argtype list on preparation, we need
+	 * If there was an argtype list on preparation, we need
 	 * an argument value list now
 	 ************************************************************/
 	if (qdesc->nargs > 0)