[illumos-Developer] Webrev: support for MHIOC_CLEAR

[Garrett D'Amore]

This contribution comes from Cyril at opensvc.com, and adds support for
missing MHIOC_CLEAR.

http://mexico.purplecow.org/gdamore/webrev/scsi-clear/

I'd like feedback... I've reviewed and it looks sane, and the test
results Cyril has gotten seem sane, so unless someone objects, this code
is likely to integrate.

(Apparently opensvc has userland tools that use this functionality.)

	- Garrett