Update contrib/check-config.sh to check for CGROUP_FREEZER and to use /proc/mounts

CGROUP_FREEZER is newly required and we should use `/proc/mounts` for checking if we have a cgroup hierarchy instead of using `/proc/$$/mountinfo` (which on systemd might not list the cgroup hierarchy)

/cc @lk4d4 wanna test this for me on your machine that was broken with ` /proc/$$/mountinfo`? :)