procfs: add pidns= mount option

[cyphar]

At the moment, procfs automatically assumes task_active_pid_ns(current) when you create a new filesystem, but it would be really handy for container runtimes to be able to specify the PID namespace manually. It would also make creating empty procfs instances (to avoid mnt_too_revealing restrictions) much easier.

I'm not sure if we would use this in libpathrs, but this is an obvious sister feature to #216.