MFC r351318, r351330, r351393, r351398, r351440, r351461, r351548, r352226, r352229

MFC r351318, r351330, r351393, r351398, r351440, r351461, r351548, r352226, r352229

r351318:
ping: Add tests of the Internet checksum function

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21340

r351330:
ping: do reverse DNS lookup of the target address

When printing replies, ping will now attempt a reverse DNS lookup of the
target.  That can be suppressed by using the "-n" option.  Curiously, ping
has always done reverse lookups in certain error paths, but never in the
success path.

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21351

r351393:
ping: add a basic functional test

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Sponsored by:	Google, inc. (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21289

r351398:
ping: By default, don't reverse lookup IP addresses

ping's default is now not to attempt reverse DNS lookups.  The -H flag will
enable them.  This change is not quite a reversion of r351330.  That change
made the happy path and error path do reverse lookups consistently; this
change changes the default for both paths.

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Discussed with:	cem
MFC-With:	351330
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21364

r351440:
ping: Fix alignment errors

This fixes -Wcast-align errors when compiled with WARNS=6.

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21327

r351461:
ping: fix unaligned access to ancillary data

Use CMSG_FIRSTHDR rather than assume that an array is correctly aligned.
Fixes warnings on sparc64 and powerpcspe.

Submitted by:	Ján Sučan <sucanjan@gmail.com>
MFH:		2 weeks
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21406

r351548:
ping: raise WARNS level to 6

Submitted by:	Ján Sučan <sucanjan@gmail.com>
Sponsored by:	Google LLC (Google Summer of Code 2019)
Differential Revision:	https://reviews.freebsd.org/D21405

r352226:
ping: fix a string in an error message

r352229:
ping: Verify whether a datagram timestamp was actually received.

ping(8) uses SO_TIMESTAMP, which attaches a timestamp to each IP datagram at
the time it's received by the kernel.  Except that occasionally it doesn't.
Add a check to see whether such a timestamp was actually set before trying
to read it.  This fixes segfaults that can happen when the kernel doesn't
attach a timestamp.

The bug has always existed, but prior to r351461 it manifested as an
implausible round-trip-time, not a segfault.

Reported by:	pho
MFC-With:	351461