The Abind case in namec() needs to cunique() the chan
before attaching the umh mount head pointer onto it.
This is because we cannot give a reference to the mount
head to any of the mh->mount...->to channels, as they
will never go away until the mount head goes away.
This is a cyclic reference.
This could be reproduced with:
@{rfork n; mount -a '#s/boot' /mnt/root; bind /mnt/root /}
Also, fix memory leaks around cunique(), which can
error, leaking the mount head we got from domount().
Move the umh != nil check inside cunique().
603d9812a7