Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

hastd: use zlib's crc32 implementation.

Reviewed by: pjd
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35767

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Switch hastctl and hastd to libmd instead of openssl for sha256

Reviewed by: bapt, brnrd, pjd
Approved by: pjd (maintainer)
Sponsored by: ScaleEngine Inc., FOSDEM DevSummit
Differential Revision: https://reviews.freebsd.org/D9423

Simplify code by changing functions types from int to avoid, as the functions
always return 0.

Found by: Clang Static Analyzer
MFC after: 1 week

Allow to checksum on-the-wire data using either CRC32 or SHA256.

MFC after: 2 weeks