cgroupsv2 support

[dzuelke]

The buildpack currently reads available RAM from a hard-coded location: /sys/fs/cgroup/memory/memory.limit_in_bytes.

This is a cgroups v1 path; cgroups v2 use a set of different soft and hard limits exposed via /sys/fs/cgroup/mem.{min,low,high,max}.

For both v1 and v2, groups can be arranged hierarchically, so it's actually necessary to determine the group that applies to the currently running process by parsing the cgroup table for the current process in /proc.

For cgroups v1, an entry may exist for a particular controller, e.g. "memory". For v2, only a single unified hierarchy exists.

For cgroups v2, all leaf nodes always expose all effective limits for active controllers, meaning no traversal up the tree is necessary to determine a limit. For v1, such path traversal must be performed, as a child group may inherit a limit from its parent.