bintrans: Remove unused variable.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D43559

bintrans: Fix uninitialized variable.

`prev` may be used uninitialized if `body` starts with a newline.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: bapt, emaste
Differential Revision: https://reviews.freebsd.org/D43534

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

bintrans: replace the quoted printable encoder/decoder

Replace the quoted printable code with one that respects RFC2045
Add tests about this parsers and encoders, using examples from the
wikipedia page.

Reviewed by: pstef
Differential Revision: https://reviews.freebsd.org/D36314