Add C binding for accessing cgroup limits

[notable-equivalent]

Add a C binding for accessing cgroup_limits.

Since C doesn't have optional types, the function instead writes the limits to passed-in-pointers, and returns a boolean of whether the information was available. This is probably the most C-like way of doing things.

Currently I have it taking in separate pointers for each data field in cgroup_limits, to avoid making a struct definition shared between C and Rust (which seemed more complex and kind of error-prone), but either way should work

[GuillaumeGomez]

Thanks!