core: support user manager with Condition[Memory/CPU/IO]Pressure

Get the cgroup root path from the current PID, so that when
ran by the user manager we can get to the right path.
Eg: foo.slice:10% will check under:

/sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/foo.slice/cpu.pressure

Follow-up for 81513b38