rlimit-util: add pid_getrlimit() helper

rlimit-util: add pid_getrlimit() helper

This is gets the resource limits off a specified process, and is very
similar to prlimit() with a NULL new_rlimit argument. In fact, it tries
that first. However, it then falls back to use /proc/$PID/limits. Why?
Simply because Linux prohibits access to prlimit() for processes with a
different UID, but /proc/$PID/limits still works.

This is preparation to allow nspawn to run unprivileged.