Remove DIOCWLABEL operation. Doing it destroyed the purpose of having

Remove DIOCWLABEL operation.  Doing it destroyed the purpose of having
write protected areas in the first place.  This also solves an issue
bootstrapping from older versions of DragonFly since the ioctl has moved
to a different header file.

Users can write-enable the label area with disklabel -W/-N if the issue
comes up.

Note: In DragonFly 1.9 and later, the MBR and disklabel areas are only
write protected when accessed via a slice or partition.  Access to these
areas via the raw disk (e.g. ad0) is no long write-protected.

Reported-by: Hasso Tepper <hasso@estpak.ee>,
	     YONETANI Tomokazu <qhwt+dfly@les.ath.cx>