Use precise exit code on usage error

[killercup]

Most people exit their program status code 1 on an error. To differentiate between program errors, the BSD folks made a guideline with exit codes (>1) to be more specific. For example, if the user doesn't have the necessary permissions, you should exit with 77.

I'm not trying to advocate rigorously following this. I want to argue from a different perspective: If people tend to use 1 for generic errors, clap should probably not use 1, so if people want to detect usage errors, they have a chance to do so. Thus, I propose changing the exit code to 64 (or exitcode::USAGE).

It seems that only these places need to be touched:

clap/src/build/app/mod.rs

Line 1209 in eaa0700

process::exit(1);

clap/src/build/app/mod.rs

Line 1285 in eaa0700

process::exit(1);

As one might see this as a breaking change for consumers, it need to land before the 3.0 release.

[pksunkara]

Merged the relevant PR.