md5: Create md5sum, etc compatible programs

On Linux, there's a similar set of programs to ours, but that end in the
letters 'sum'. These act basically like FreeBSD versions run with the -r
option. Add code so that when the program ends in 'sum' you get the
linux -r behavior. This is enough to make most things that use sha*sum
work correctly (the -c / --check options, as well as the long args are
not implemented). When running with the -sum programs, ignore -t instead
of running internal speed tests and make -c an error.

Reviewed by: sef, and kp and allanjude (earlier version)
Relnotes: yes
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30309