Update to Rust nightly
Handle exit status more consistently. #211

This changes the `uumain` functions to return the exit code as `int`.

It does not eliminate all calls to `os::set_exit_status` and `libc::exit`.
Get test suite passing again by tweaking error handling in mkdir

Replace `show_error!` with Result<(), int> and `display_error!`
Make show_error! not set the exit code. #211
Make busytest stop complaining about missing config file
Fix typo
Restore error recovery behavior in rm, rmdir, etc.

These tools continue on error. This patch uses `Result` and the
`and` method to collect `Err` values.
Restore error handling to md5sum
Remove tail return statements.

Per review feedback.
phase(syntax) -> phase(plugin)