Bail out of acd_open() when we're called with a cdev_t that wasn't created

Bail out of acd_open() when we're called with a cdev_t that wasn't created
by us. This happened when e.g. booting from cd; 'acd0c' would then be
resolved to minor 2 instead of the minor 0 we create in acd_attach(). If
one, instead of booting the kernel through, exits to the loader prompt from
the bootloader, then issues `unload` and `load kernel`, then `boot`, this
problem wouldn't show up. My guess is, somewhere, somehow, the bootloader
is mucking about in our cdev storeroom. I haven't yet found where this
happens.