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

(cherry picked from commit 72ee91fed4cfdcfbfb767cc166370b40e50d446a)

md5: Ignore files in string and passthrough mode.

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

(cherry picked from commit 5b44edb4058365ba8e4ccfdb5176c1cddd4394fe)

md5: Clean up input stream rights.

Keep it simple, caph_limit_stdio() and fileargs_fopen() already take
care of everything for us.

MFC after: 1 week
Reviewed by: markj, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D43897

(cherry picked from commit 17d5b027c1921d0c6ba2de7993dd808dbf4df078)

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

(cherry picked from commit e7308a60a5b77a1e4aff6e27eddef40fedabb98c)

md5: Fix Perl mode long options.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp, allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43989

(cherry picked from commit 4db7ca24470576948e33f48033cfa5be2749950e)

md5: Fix GNU check mode.

Fixes: 9b20849bc5f1b500f2de7aeca77f0e6556069bbb
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp, allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43990

(cherry picked from commit c05533d97ac178f8e811407860d47d3c0ae61eed)

md5: Untabify declarations.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp, allanjude, markj
Differential Revision: https://reviews.freebsd.org/D43991

(cherry picked from commit 702f133fa18185a7e10e66316e8f158be935b696)