ramfs: If cwd is removed the curproc will be jailed

[OlliV]

If the working directory of a process is removed the process is jailed to that directory
as the vnode doesn't get freed but dot and dotdot links are deleted. For some reason
chdir to / is not working either so rmdir . effectively creates a chroot-like jail.

[OlliV]

This seems to happen mostly on ramfs. By now cd to / works ok after rmdir . but the directory is improperly destroyed.

[OlliV]

cd /tmp
mkdir d
cd d

Now rmdir /tmp/d works but rmdir . only removes dot and dotdot links.

[OlliV]

Fixed by 3395e70