/null/dev: we're interested in /dev/pts and nullfs doesn't traverse
underlying mountpoints.
(I had code for nullfs mountpoint traversal in the tree i used to
test this originally... but I assume the rest of the world doesn't.
Before this change the test would still fail, but fail in the wrong
place and due to the wrong reason.)
Note: I'm not adding these to the build yet, since they depend on
some other other cleanup I might get done only after the weekend.
Even so, t_nullpts serves a simple example of how to repeat the
crash described in PR kern/43456 (just remove "rump_sys_" from the
calls and it should compile and you should get a host kernel panic
instead of a coredump).
