If mount fails, let's "modprobe" the partition type and try again

It would appear that mount fails to "modprobe btrfs" for whatever reason.  This is a reasonably clean workaround for that problem (modprobe $PARTITION_TYPE), and I've only applied it explicitly when "mount" fails the first time.

Also, instead of doing `blkid -o device -t ... | head -n1`, we can do `blkid -o device -l -t ...`:
```console
$ blkid -h
...
 -l          look up only first device with token specified by -t
...
```