Options parsing: `-p` in `--ocaml` leads to bogus error

[andreasabel]

With misspelled option (-p instead of -d)

bnfc --ocaml -m -p Issue204.cf 

reports

Missing grammar file
usage: bnfc [--version] [--help] <target language> [<args>] file.cf

However, -p is not a valid option for the ocaml backend.