md5: Add test case for GNU input modes.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43988

md5: Ignore files in string and passthrough mode.

MFC after: 1 week
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43871

md5: Accept "-" as alias for stdin.

(based on a patch by jhb)

MFC after: 1 week
PR: 276915
Reported by: Hannes Hauswedell <h2+fbsdports@fsfe.org>
Reviewed by: allanjude, markj, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D43870

md5: Improve compatibility.

* Overhaul the GNU compatibility mode to more closely emulate what the GNU tools do.

* Add a Perl compatibility mode which emulates the shasum tool that ships with Perl. This is currently not installed.

* Overhaul the tests.

Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D39446

md5: fix *sum -c with missing files

If we fail to open one of the files in the file listing, we still need
to advance `rec` along with `argv` so that the checksum we're checking
against lines up with the file we're hashing.

Tests added both for the -c flag, as well as the -b and -t modes of
the *sum programs.

PR: 267722
Reviewed by: emaste (earlier version)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D37374