Unprivileged user namespaces are not allowed on Ubuntu 24.04

[stevenengler]

Ubuntu 24.04 (any maybe 23.10) requires AppArmor profiles for programs that use unprivileged user namespaces. This means that socksns panics.

Example:

$ socksns bash
panicked at /path/socksns-0.3.3/src/main.rs:52:71:
called `Result::unwrap()` on an `Err` value: EPERM
panicked after panic::always_abort(), aborting.

The README should document this.