These used to live in TODO but we got rid off them. Make sure
we keep track of what we want to support by printing a message
when those flags are unimplemented.
POSIX uniq(1) is required to write to its second argument,
if one is given.
The multiple-input feature I accidentally added might be useful,
but users and scripts who rely on it would be put at risk for data loss
if they ever run into a POSIX uniq(1).
