midicat(1): set ifile/ofile to "stdin"/"stdout" if it is an en-dash ("-")

midicat(1): set ifile/ofile to "stdin"/"stdout" if it is an en-dash ("-")

This makes error messages a little bit more intuitive.  Instead of, e.g.:

midicat: -: No space left on device

you get:

midicat: stdout: No space left on device

Link: https://marc.info/?l=openbsd-tech&m=166982129428027&w=2
ok millert@ kn@ ratchov@