This adds a hidden `completion` subcommand to coreutils. When invoked with
`coreutils completion <utility> <shell>` a completion file will be printed
to stdout. When running `make install` those files will be created for all
utilities and copied to the appropriate locations.
`make install` will install completions for zsh, fish and bash; however,
clap also supports generating completions for powershell and elvish.
With this patch all utilities are required to have a publich uu_app function
that returns a clap::App in addition to the uumain function.
New tests in busybox are based on the fact that the function
appears in the usage of the busybox binary.
Because the tests are searching for an exact string they don't see
the function defined by coreutils.
By using the exact same string as busybox we can now also run the new
busybox tests
