Change a few of the more common disk drivers (sd, cd, wd, rd, and vnd)

to return EBUSY if the user tries to modify an open partition's offset
or size.  Only sadness can result if a user tries this, and rejecting
it prevents a race between sdstart() and sdstrategy().

Curiously, there was already code in the kernel and in disklabel(8) to
detect/handle this, but it was effectively disabled because the disk
drivers always used something like "/* sc->sc_dk.dk_openmask */ 0",
and this commented out code has existed since even r1.1 in NetBSD.

I had no problems building a release and messing around with
disklabel(8) for a bit with this diff.  Canarying the more common MI
disk drivers until we gain confidence that there aren't any
regressions, then we can switch the remaining drivers.

"I am surprised you got me convinced that this stuff is safe" deraadt@
ok krw@

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