cntr exec fails with umask set to 077

[JustWhateverIOnlyWantBetterCodeView]

To reproduce run umask 077 in your shell and then attach to any container running non-root user inside. After that try to run cntr exec. It fails, because /.cntr/pid file was created with permissions 600 and is not readable.

I've tested with zsh and bash.

Also with umask set to 000 the pid file is created with permissions 666 which means it's writable by anyone, which could potentially be a security issue?

To fix this the correct permissions should be set when this file is created:

cntr/src/dotcntr.rs

Lines 26 to 37 in 43f88bf

	pub fn write_pid_file(&self, target_pid: Pid) -> Result<()> {
	let path = self.dir.path().join("pid");
	let mut file = try_with!(File::create(&path), "failed to create {}", path.display());
	let raw_pid: pid_t = target_pid.into();
	try_with!(
	file.write_all(format!("{}", raw_pid).as_bytes()),
	"failed to write {}",
	path.display()
	);
	Ok(())
	}