Make `quot -a' work when we've got slashes in the device name.
Make `quot -a' work when we've got slashes in the device name.

A very long time ago we had raw device nodes. quot(8) was supposed to
use these  when running `quot -a'. For some reason the code got once
changed to strip the device name until it reaches the last slash. This
is not reliable, because this means /dev/mirror/foo will be stripped to
/dev/foo.

This bug also exists on RELENG_7 and RELENG_6, but I think I'll just
merge them back somewhere after the upcoming releases. There's no rush.

MFC after:	2 months

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