Let Disk_Names() take advantage of the kern.disks sysctl if it's available.  If not, it falls back to the existing hack and slash method.
Let Disk_Names() take advantage of the kern.disks sysctl if it's
available.  If not, it falls back to the existing hack and slash method.

A positive side effect is that non-root users may now use Disk_Names(),
for non-dangerous libh/disk.tcl testing.

Reviewed by:	phk

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase