Refactor the various delete methods out of dastart().  Cleans up a bunch
of style and adds more modularity and clarity.

Obtained from:	Netflix
MFC after:	3 days

(cherry picked from commit 1a38eadae75759280e35b93ed5683f224eaed451)
Const-ify the new da_delete_functions.
Remove a redundant sanity check

Submitted by:	Steven Hartland
Obtained from:	Netflix
MFC after:	3 days

(cherry picked from commit bfbf95a9a25ffe82c3ee1ca6f2c6ffaf70edd43d)
Correct short delete issue in SCSI UNMAP support
Correct missing \n's in xpt_print's
Correct incorrect count being passed to short delete xpt_print

(cherry picked from commit db5c674847aba156ed1cc192559deabc12d290b4)

Ticket:	#3864
Allow delete_method sysctl to be set to "DISABLE".

(cherry picked from commit 2fd0db3dfd7f379b8a76e72f2ce8171b55efe1c7)