spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Add an internal libiscsiutil library.

Move some of the code duplicated between ctld(8) and iscsid(8) into a
libiscsiutil library.

Sharing the low-level PDU code did require having a
'struct connection' base class with a method table to permit separate
initiator vs target behavior (e.g. in handling proxy PDUs).

Reviewed by: mav, emaste
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33544