md5: Treat a filename of '-' as stdin

[jhb]

Summary

PR: 276915
Reported by: Hannes Hauswedell <[e-mail]>

[imp]

Do other md5/md5sum have this feature?
Does it need a mention in the man page?
Assuming yes,no, I'm good. But I'm also good with you sorting out the impact of my questions if there is any

[jhb]

The bug noted that GNU's md5sum permits - as a filename for stdin.

[emaste]

man page update?

[des]

I'll take care of the man page and tests in a separate commit.

[des]

sbin/md5/md5.c

641

Should we check that f != stdin here? If we do, md5 - - foo will print the digest of whatever is on stdin, then the digest of the empty string, then the digest of foo. If we don't, it will print the digest of whatever is on stdin and then error out on EBADF without looking at foo.

[des]

D43870 has this + fclose() check + doc and test changes.