[9fans] Fileserver doesn't exit when it should

[Porlock <porlock@mailinator.com>]

Here is some behaviour which I don't understand.  Something seems to be
preventing a fileserver process from exiting (since it doesn't receive
end-of-file on its 9p pipe).

To see this, open a new rio window and enter the following commands :-

ramfs -m /n/junk
cp /bin/echo /n/junk
/n/junk/echo hello
unmount /n/junk

Now try to close the rio window, by typing ^D at the prompt.  The
window will not close.  Further, the ramfs server is still running.
It must be killed manually (from another window), and then the window
will close.

I don't think this has anything to do with ramfs, since I tried the
same commands with a completely different ram filesystem server, and
it exhibited the same behaviour.

This behaviour doesn't happen on 9front, which behaves as you would
expect, with the file server exiting on unmount, and the window
closing on ^D.