Our MBR/superblock/LVM/ZFS smashing code in rc.frisbee relied on dmesg
output to determine the local disks to call zapdisk on. However, the RE we
used assumed well ordered output like:

  da0 at mpt0 bus 0 scbus0 target 0 lun 0
  da0: <ATA WDC WD5003ABYZ-0 1S03> Fixed Direct Access SPC-3 SCSI device
  da0: Serial Number      WD-WMAYP0DPNFLM
  da0: 300.000MB/s transfers
  da0: Command Queueing enabled
  da0: 476940MB (976773168 512 byte sectors)

where we matched that last line. But due to the asynchronous nature of disk
initialization, probably due to some soon-to-be-failing disks on the d710s,
the last line was delayed and came out mashed-up with the da1 output:

  da1: <ATA WDC WD5003ABYX-1 1S02> Fixed Direct Access SPC-3 SCSI device
  da1: Serial Number      WD-WMAYP4939538
  da1: 300.000MB/s transfersda0: 476940MB (976773168 512 byte sectors)

so we didn't see da0 and didn't call zapdisk on it. This led to some LVM
metadata on /dev/sda4 leaking through to a new experiment and if that experiment
tried to setup LVM (e.g., a vnode host), it would blow up.

Now we use a sysctl call (kern.disks) to get the disk names.