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: 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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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: Ensure that tftp_log() preserves errno.

Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D38956

tftpd: whitespace cleanup

tftpd: cleanup

Sponsored by: Klara, Inc.

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

Fix several Coverity warnings in tftp

Some of the changes are in the libexec/tftpd directory, but to functions that
are only used by tftp(1) (they share some code).

* strcpy => strlcpy (1006793, 1006794, 1006796, 1006741)
* Unchecked return value and TOCTTOU (1009314)
* NULL pointer dereference (1018035, 1018036)

Reported by: Coverity
CID: 1006793, 1006794, 1006796, 1006741, 1009314, 1018035
CID: 1018036
MFC after: 2 weeks

Fix multiple Coverity warnings in tftpd(8)

* Initialize uninitialized variable (CID 1006502)
* strcpy => strlcpy (CID 1006792, 1006791, 1006790)
* Check function return values (CID 1009442, 1009441, 1009440)
* Delete dead code in receive_packet (not reported by Coverity)
* Remove redundant alarm(3) in receive_packet (not reported by Coverity)

Reported by: Coverity
CID: 1006502, 1006792, 1006791, 1006790, 1009442, 1009441, 1009440
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D11287

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.

Fix warnings found by -Wmising-variable-declarations.

This self-written compiler warning, which is hopefully going to be
committed into LLVM sources soon, warns about potentially missing
`static' keywords, similar to -Wmissing-prototypes.

- bin/pax: Move external declaration of chdname and s_mask into extern.h.
- bin/setfacl: Move setfacl.c-specific stuff out of setfacl.h.
- sbin/mount_fusefs: Remove char *progname; use getprogname().
- others: add `static' where possible.

Fix typos.

PR: bin/148894
Submitted by: olgeni

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