Make softraid helper return disk not device names

Make softraid helper return disk not device names

get_softraid_chunks() yields full device names with labels, e.g. "sd0a",
but the installer largely deals with disk names "sd0".

The only user of this, MAKEDEV, behaves identically with either form.

Rename to get_softraid_disks(), clarify comments and return disk names
so it can be reused without pattern matching or stripping labels.

OK afresh1