scx_layered: Make layered work in pid namespaces

[hodgesds]

Add an additional helper function to translate the scx_layered userspace tgid when running in a pid namespace.

Tested by running layered in a separate pid namespace:

$ cargo build --release && sudo unshare --mount-proc --fork -p ./target/release/scx_layered --run-example --stats 1 -vvv

Output from trace:

$ sudo bpftool prog trace | grep tgid
           <...>-242372  [061] ....1 3574039.989305: bpf_trace_printk: CFG layered running with tgid: 242372
$ pidof scx_layered
242372