Stopgap patch for problem with cursors using hash joins: an ABORT

Stopgap patch for problem with cursors using hash joins: an ABORT
would cause two attempts to delete the hashtable portal, one from
query shutdown and one from portalmem.c.  Fix by making hash portals
be treated as 'special' portal names, so that CollectNamedPortals
will not think it should delete them.  This code is in need of
complete rewrite (and is already largely rewritten in current
sources), but still need to put a finger in the dike for 7.0.*.