uniq: allow -c to be used with -d or -u

Bring in some bits from NetBSD and lift the restriction in uniq(1) that
-c cannot be used with the -d and -u options. This restriction seems
unnecessary and is supported at least by GNU, OpenBSD, and NetBSD. Lift
the restriction and simplify the show() logic a little bit to maintain
functionality when -c is provided with -d/-u.

Also with this change, -d and -u are now actually a mutually exclusive,
albeit valid, combination. Given that they both indicate opposite
behavior, uniq(1) will no longer output anything if both -d and -u are
supplied. This is in line with NetBSD as well as GNU.

Adjust the man page and usage() to reflect that -c is its own standalone
option.

PR: 200553
Submitted by: Kyle Evans <kevans91@ksu.edu>
Reviewed by: cem, emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10694