Fix Linux mkextrafs support for unpartitioned device for new sfdisks.
Fix Linux mkextrafs support for unpartitioned device for new sfdisks.

sfdisk no longer accepts any other unit than sector.  Ha, ha.

Also, warn user if they try to mkextrafs on an unpartitioned disk and
fail to supply '-s 1'.  We only generate partition 1, full disk size
(minus sectors 0-2048) in this case.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome