retry mounting with ro when rw fails

[Matthew Jacob]

> The first part is in CAM/SCSI to make sure that ENODEV is consistently returned to
> signal that an operation is not supported by a device (in accordance to intro(2))
> and specifically to return ENODEV on write attempt to a read-only or
> write-protected media.  Making this change in SCSI should cover real SCSI devices,
> as well as ATAPI through ahci/siis/atapicam or similar, plus majority (all?) of
> USB Mass Storage devices.

This may be the right thing to do (which is debatable) but I can tell 
you here and now this will break a lot of commercial usages of FreeBSD 
with SANS which depend (foolishly) on existing error codes.