xargs doesn't limit command line lengths properly

[tavianator]

$ find ~ -print0 | ./target/debug/xargs -0 echo >/dev/null
Error: Command could not be run: Argument list too long (os error 7)

From a quick look, there's at least an issue here:

findutils/src/xargs/mod.rs

Lines 141 to 146 in 36e3229

	#[cfg(unix)]
	fn count_osstr_chars_for_exec(s: &OsStr) -> usize {
	use std::os::unix::ffi::OsStrExt;
	// Include +1 for the null terminator.
	s.as_bytes().len() + 1
	}

This needs to count not only the length of the string, but also the size of the pointer that ends up in argv/envp.

It might be worth using the https://crates.io/crates/argmax crate to do this accounting for us. It could also help for #6.