Skip softraid(4) keydisks silently

Skip softraid(4) keydisks silently

Logging the presence of a keydisk the same way offline data chunks are
logged seems unjustified:

Offline data chunks mean the softraid volume is degraded and installboot(8)
should be rerun when they're online.

Offline keydisks just means the user unplugged their USB key or so and
installboot must never touch them anyway, so the absence of keydisks is
meaningless to installboot -- it should never touch them.

So a) drop the "is keydisk - skipping" message and b) hoist the keydisk
check before the offline check so as to avoid "not online - skipping"
messages for offline keydisks.