1. make fileops const
2. add 2 new negative errno's to `officially' support the cloning hack:
- EDUPFD (used to overload ENODEV)
- EMOVEFD (used to overload ENXIO)
3. Created an fdclone() function to encapsulate the operations needed for
EMOVEFD, and made all cloners use it.
4. Centralize the local noop/badop fileops functions to:
fnullop_fcntl, fnullop_poll, fnullop_kqfilter, fbadop_stat