When booting from CD, check cd1a and acd1a after cd0a and acd0a, allowing

When booting from CD, check cd1a and acd1a after cd0a and acd0a, allowing
a CD to be booted off of a second CDRom drive.

When checking for the same rootdev selection as already has been set,
only the major number of the existing rootdev was being checked.  This
prevent other unit numbers from being tried properly (e.g. trying cd1 after
having already tried cd0).

Completely parse the rootdev's (unit, slice, partition) tuple before
trying to look up the device, rather then assuming that devname(0,0,0)
will exist (this is no longer necessarily true).