tftpd: Satisfy clang-analyzer.

* Replace `random()` with `arc4random()`.
* Change some variable types.
* Drop some unused assignments.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D45132

tftpd: Drop unneeded includes.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D45130

tftpd: Use `size_t` where appropriate.

* Limit the use of `ssize_t` to only where it's needed.
* Correct one case of `int` being used for a length.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44954

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

tftpd: whitespace cleanup

Add support for the TFTP windowsize option described in RFC 7440.

The windowsize option permits multiple blocks to be transmitted
before the receiver sends an ACK improving throughput for larger
files.

Reviewed by: asomers
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23836

libexec: 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.

Mark tftp_log() as __printflike() (which would have caught the bug
fixed in r246106) and deal with the fallout.

MFC after: 2 weeks

Spelling fixes for libexec/

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Make WARNS=6 clean.

MFC after: 1 week

Bring in new files from edwin's tftp