spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

iscsi: per-session timeouts and rapid teardown of session on reconnect

Add per-Session configurable ping (SCSI NOP) and login timeouts.

Remove the torn down, old iSCSI session quickly, when performing a reconnect.

Reviewed By: trasz
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34198

iscsi: Handle large Text responses.

Text requests and responses can span multiple PDUs. In that case, the
sender sets the Continue bit in non-final PDUs and the Final bit in
the last PDU. The receiver responds to non-final PDUs with an empty
text PDU.

To support this, add a more abstract API in libiscsi which accepts and
receives key sets rather than PDUs. These routines internally send or
receive one or more PDUs. Use these new functions to replace the
handling of TextRequest and TextResponse PDUs in discovery sessions in
both ctld and iscsid.

Note that there is not currently a use case for large Text requests
and those are still always sent as a single PDU. However, discovery
sessions can return a text response listing targets that spans
multiple PDUs, so the new API supports sending and receiving multi-PDU
responses.

Reported by: Jithesh Arakkan @ Chelsio
Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33548

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: 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