Support reading cpu.max from cgroups v2 (#125)

Author: OrionNebula

Diff for: fixtures/cgroups2/cgroups/ceil/cpu.max

@@ -0,0 +1 @@
+150000 100000

Diff for: fixtures/cgroups2/cgroups/good/cpu.max

@@ -0,0 +1 @@
+600000 100000

Diff for: fixtures/cgroups2/cgroups/zero-period/cpu.max

@@ -0,0 +1 @@
+600000 0

Diff for: fixtures/cgroups2/proc/cgroups/cgroup

@@ -0,0 +1,2 @@
+12::/
+3::/user.slice

Diff for: fixtures/cgroups2/proc/cgroups/cgroup_multi

@@ -0,0 +1,3 @@
+12::/
+11:cpu,cpuacct:/
+3::/user.slice

Diff for: fixtures/cgroups2/proc/cgroups/mountinfo

@@ -0,0 +1,5 @@
+1 0 8:1 / / rw,noatime shared:1 - ext4 /dev/sda1 rw,errors=remount-ro,data=reordered
+2 1 0:1 / /dev rw,relatime shared:2 - devtmpfs udev rw,size=10240k,nr_inodes=16487629,mode=755
+3 1 0:2 / /proc rw,nosuid,nodev,noexec,relatime shared:3 - proc proc rw
+4 1 0:3 / /sys rw,nosuid,nodev,noexec,relatime shared:4 - sysfs sysfs rw
+5 4 0:4 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime shared:5 - cgroup2 cgroup2 rw,nsdelegate,memory_recursiveprot