MFC r361688-r361690, r365041

r361688:
Restore compatibility with GNU diff regarding --label

Various options to "diff(1)" show filenames, and traditionally make use of the
"--label" parameter, if set.

Restore this behaviour in BSD diff.

While here add a regression test

PR: 244533
Submitted by: Jamie Landeg-Jones <jamie@catflap.org>

r361689:
diff: restore compatibility with GNU diff regarding -N option

When -N is used the missing files are treated as empty.

PR: 233402
Submitted by: Fehmi Noyan Isi <fnoyanisi@yahoo.com>
Reported by: Roman Neuhauser <roman@sigpipe.cz>
Differential Revision: D25081

r361690:
Document long version of -b option

PR: 234195
Submitted by: Fehmi Noyan Isi <fnoyanisi@yahoo.com>
Reported by: Andras Farkas <deepbluemistake@gmail.com>

r365041:
diff: always properly kill pr(1)

When diff is invoked with -l it will spawn the pr(1) program.
In some circumpstances the pr(1) was not properly killed when diff program
exits.

Submitted by: Bret Ketchum
Differential Revision: https://reviews.freebsd.org/D26232