Enhancement: `std::env::progname()`

It is often desirable to include a short name for the program in error messages. The way for a program to learn its own name, right now, is rather verbose:

fn progname() -> String {
    let argv0 = env::args_os().next().unwrap();
    let exec = Path::new(&argv0).file_name().unwrap().to_str().unwrap();

    String::from(if cfg!(windows) && exec.ends_with(".exe") {
        exec.split_at(exec.len() - 4).0
    } else {
        exec
    })
}

And it gets worse if you want to be diligent about the error handling.

I propose that we add std::env::progname which is the above operation with the following refinements:

• It does not panic. It does not return Result either. It returns a predictable fixed string (e.g. "<program name unknown>") if it can't determine the name by which the program was invoked.

• For efficiency, the result of the above path-bashing is memoized. The return type is &'static str.

Start by reviewing the proposed std::env::progname API and the example implementation in the issue. Determine the appropriate Rust standard-library entry point and behavior for unavailable program names, Windows executable suffixes, memoization, and a &'static str return; the work is done when those decisions are implemented and covered by tests.