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, and likely OpenBSD but this has not been checked.
Adjust the man page and usage() to reflect that -c is its own standalone option.
This is a partial revert of r98547.