uniq: Correctly document the -D option.

The -D option takes an optional argument; modify the usage message
and the manual page's synopsis to correctly reflect this. Also
update the tests to exercise -D with and without an argument.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D47999