Make SELECT FOR UPDATE/SHARE work on inheritance trees, by having the plan

Make SELECT FOR UPDATE/SHARE work on inheritance trees, by having the plan
return the tableoid as well as the ctid for any FOR UPDATE targets that
have child tables.  All child tables are listed in the ExecRowMark list,
but the executor just skips the ones that didn't produce the current row.

Curiously, this longstanding restriction doesn't seem to have been documented
anywhere; so no doc changes.