Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r243314:
Zero the whole struct not just the size of a pointer.

MFC r246670:
Major update for unix_cmsg.

PR: bin/131567
Submitted by: Andrey Simonenko <simon@comsys.ntu-kpi.kiev.ua>

MFC r241844:
remove duplicate semicolons where possible.

Approved by: cperciva (implicit)

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

o Let getopt(3) report errors in command line arguments.
o If something is wrong with options, then output short usage help message.
o Output errstr returned from strtonum(3).

PR: bin/98141
Submitted by: Andrey Simonenko

o Add a collection of regression tests for ancillary (control)
data passing for unix domain sockets, stream and datagram.

There are 15 tests:

Test/Type of socket STREAM DGRAM
----------------------------------------------------------------------
Sending, receiving cmsgcred 1 6
Receiving sockcred (listening socket has LOCAL_CREDS) 2 n/a
Receiving sockcred (accepted socket has LOCAL_CREDS) 3 n/a
Receiving sockcred n/a 7
Sending cmsgcred, receiving sockcred 4 8
Sending, receiving timestamp 5 9
Sending, receiving cmsgcred (no control data) 10 13
Sending cmsgcred, receiving sockcred (no control data) 11 14
Sending, receiving timestamp (no control data) 12 15

Currently we pass 8 tests. All the rest marked as TODO.

PR: kern/90800
Submitted by: Andrey Simonenko