Improve ALTER PUBLICATION validation and error messages

Attempting to add a system column for a table to an existing publication
would result in the not very intuitive error message of:

ERROR:  negative bitmapset member not allowed

Here we improve that to have it display the same error message as a user
would see if they tried adding a system column for a table when adding
it to the publication in the first place.

Doing this requires making the function which validates the list of
columns an extern function.  The signature of the static function wasn't
an ideal external API as it made the code more complex than it needed to be.
Here we adjust the function to have it populate a Bitmapset of attribute
numbers.  Doing it this way allows code simplification.

There was no particular bug here other than the weird error message, so
no backpatch.

Bug: #18558
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Peter Smith, David Rowley
Discussion: https://postgr.es/m/18558-411bc81b03592125@postgresql.org

src/backend/commands/subscriptioncmds.c

@@ -2266,7 +2266,7 @@ fetch_table_list(WalReceiverConn *wrconn, List *publications)
 		 *
 		 * Note that attrs are always stored in sorted order so we don't need
 		 * to worry if different publications have specified them in a
-		 * different order. See publication_translate_columns.
+		 * different order. See pub_collist_validate.
 		 */
 		appendStringInfo(&cmd, "SELECT DISTINCT n.nspname, c.relname, gpt.attrs\n"
 						 "       FROM pg_class c\n"