As per Utility Syntax Guidelines, accept both forms: -l -n and -ln.
To do that, anticipate the source string for the next option that will
be parsed by nextopt(). It's not always *argptr, sometimes it is
nextopt_optptr.
To simplify the check for not_fcnumber, slightly modify nextopt() to
always nullify nextopt_optptr in cases where it would have been set
to point to a NUL character.
Maybe also s/shows in/uses/ for consistency with the previous sentence, but that's a matter of taste. The typo itself is the important thing.