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