Capsicum support for dd(1)

Adds Capsicum sandboxing to dd utility.

Submitted by: Pawel Biernacki <pawel.biernacki@gmail.com>
Reviewed by: allanjude, emaste, oshogbo
Approved by: oshogbo
Sponsored by: Mysterious Code Ltd.
Differential Revision: https://reviews.freebsd.org/D8543