cat(1): refactor cook_args()/raw_args() into single function, cat_file()

cat(1): refactor cook_args()/raw_args() into single function, cat_file()

- Combine the open/close portions of cook_args()/raw_args() into a single
  function, cat_file().

- Push the flag-checking conditional in main() down into cat_file().

- Pull the argv loop in cat_file() up into main().

These changes -- especially pulling the argv look up into main() --
will allow us to drop the "rpath" promise in a single spot in a
subsequent patch.

Tweaked by mestre@.  Descriptor leak in earlier version spotted by
Matthew Martin.

Thread: https://marc.info/?l=openbsd-tech&m=163941848104274&w=2

No objections on tech@ after several weeks.