Fix booting from an IDE block device on the Sun Blade 100. Apparently

Fix booting from an IDE block device on the Sun Blade 100.  Apparently
writing to disk using the Open Firmware interfaces is buggy and causes
corruption of the disk.  While it isn't entirely clear what versions
of Open Firmware are affected, but it seems to only affect IDE drives.
So if we detect an IDE drive, disable writing to it.  This results in
a small lose of bootloader functionality (bsd.upgrade loop prevention
and flagging /etc/random.seed re-use) but that is better than losing
the ability to run OpenBSD at all.

Based on a diff by Ted Bullock (who did all the hard work of debugging
this and coming up with a viable fix).

ok deraadt@