devel/bitkeeper: fix poudriere build

r569286 fixed manual port stage however it broke poudriere builds.
This resulted in either a nested directory tree of
$STAGEDIR/$PREFIX/bitkeeper/$PREFIX/bitkeeper under poudriere or simply
$STAGEDIR when building by hand, making any conventional fix mutulally
exclusive. To resolve we rename DESTDIR within the port to some arbitrary
name, PDESTDIR, and set the port's BINDIR to /bitkeeper, from
/usr/local/bitkeeper, to avoid the other source of nesting.