libcam: Define depop structures and introduce scsi_wrap

Define structures related to the depop set of commands (GET PHYSICAL ELEMENT
STATUS, REMOVE ELEMENT AND TRUNCATE, and RESTORE ELEMENT AND REBUILD) as
well as the CDB construction routines.

Also create scsi_wrap.c. This will have convenience routines that will do all
the elements of allocating the ccb, generating the CDB, sending the command
(looping as necessary for cases where data is returned, but it's size isn't
known up front), etc. As this functionality is fleshed out, calling many
camcontrol commands programatically gets much easier.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D29017