spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

pkg(7): now that we do use libmd, use it completly

Use SHA256_Fd and SHA256_Data instead of home made equivalent.
wrap those functions into hash.c to avoid header collition between
openssl and libmd

Suggested by: kevans

pkg(7): use libmd for sha256 instead of openssl

OpenSSL 3.0 has deprecated the sha256 api, let's use libmd which has the
same API instead.

In order to avoid the collision in definitions (sha256.h cannot be
included in the same file as a file where openssl headers has been
included) let's move the sha256 related code in its own file

PR: 270023
Reported by: ngie