use XDG_RUNTIME_DIR instead of /tmp

The issue is, that POSIX only enforces the permissions of the containing
directory but not the permissions of the socket itself. This can lead to
potential security issues, as the command socket is not built for untrusted
input. In the case that `XDG_RUNTIME_DIR` is not available, a directory under
`/tmp` is created having the correct permissions to protect the socket under
it.