Back-patch some plpython patches previously made only in HEAD: changes of

Back-patch some plpython patches previously made only in HEAD: changes of
3-Apr and 4-Apr to declare interface functions properly and eliminate casts,
thereby fixing potential problems on 64-bit machines; and changes of 13-Jul
to volatile-qualify some variables to suppress compiler warnings.
Per discussion, we're only worrying about Python 2.5 in PG 8.2 and up,
so no need to patch further back.