md5(1): fix -c flag to work with input on stdin

Previously, the -p and -c flags were ignored when reading from stdin
Additionally, -s and -c can be used together now.

PR: 247295
Reviewed by: kevans
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25362