spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

libiscsiutil: Fix a typo in a source code comment

- s/sesion/session/

MFC after: 3 days

libiscsiutil: Change keys_load/save to operate on data buffers.

This will be used in future changes to support large text requests
spanning multiple PDUs.

Provide wrapper functions keys_load/save_pdu that operate use a PDU's
data buffer.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33547

libiscsiutil: Use open_memstream to build the outgoing block of keys.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33546

libiscsiutil: Fix a memory leak with negotiation keys.

When keys are loaded from a received PDU, a copy of the received keys
block is saved in the keys struct and the name and value pointers
point into that saved block. Freeing the keys frees this block.

However, when keys are added to a keys struct to build a set of keys
later sent in a PDU, the keys data block pointer is not used and
individual key names and values hold allocated strings. When the keys
structure was freed, all of these individual key name and value
strings were leaked.

Instead, allocate copies of strings for names and values when parsing
a set of keys from a received PDU and free all of the individual key
name and value strings when deleting a set of keys.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33545

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