core: add support for the "pids" cgroup controller

This adds support for the new "pids" cgroup controller of 4.3 kernels.
It allows accounting the number of tasks in a cgroup and enforcing
limits on it.

This adds two new setting TasksAccounting= and TasksMax= to each unit,
as well as a gloabl option DefaultTasksAccounting=.

This also updated "cgtop" to optionally make use of the new
kernel-provided accounting.

systemctl has been updated to show the number of tasks for each service
if it is available.

This patch also adds correct support for undoing memory limits for units
using a MemoryLimit=infinity syntax. We do the same for TasksMax= now
and hence keep things in sync here.

src/basic/cgroup-util.c

@@ -2018,9 +2018,10 @@ int cg_mask_supported(CGroupMask *ret) {
                         mask |= CGROUP_CONTROLLER_TO_MASK(v);
                 }
 
-                /* Currently, we only support the memory controller in
-                 * the unified hierarchy, mask everything else off. */
-                mask &= CGROUP_MASK_MEMORY;
+                /* Currently, we only support the memory and pids
+                 * controller in the unified hierarchy, mask
+                 * everything else off. */
+                mask &= CGROUP_MASK_MEMORY | CGROUP_MASK_PIDS;
 
         } else {
                 CGroupController c;
@@ -2212,6 +2213,7 @@ static const char *cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
         [CGROUP_CONTROLLER_BLKIO] = "blkio",
         [CGROUP_CONTROLLER_MEMORY] = "memory",
         [CGROUP_CONTROLLER_DEVICES] = "devices",
+        [CGROUP_CONTROLLER_PIDS] = "pids",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(cgroup_controller, CGroupController);

src/basic/cgroup-util.h

@@ -35,6 +35,7 @@ typedef enum CGroupController {
         CGROUP_CONTROLLER_BLKIO,
         CGROUP_CONTROLLER_MEMORY,
         CGROUP_CONTROLLER_DEVICES,
+        CGROUP_CONTROLLER_PIDS,
         _CGROUP_CONTROLLER_MAX,
         _CGROUP_CONTROLLER_INVALID = -1,
 } CGroupController;
@@ -48,6 +49,7 @@ typedef enum CGroupMask {
         CGROUP_MASK_BLKIO = CGROUP_CONTROLLER_TO_MASK(CGROUP_CONTROLLER_BLKIO),
         CGROUP_MASK_MEMORY = CGROUP_CONTROLLER_TO_MASK(CGROUP_CONTROLLER_MEMORY),
         CGROUP_MASK_DEVICES = CGROUP_CONTROLLER_TO_MASK(CGROUP_CONTROLLER_DEVICES),
+        CGROUP_MASK_PIDS = CGROUP_CONTROLLER_TO_MASK(CGROUP_CONTROLLER_PIDS),
         _CGROUP_MASK_ALL = CGROUP_CONTROLLER_TO_MASK(_CGROUP_CONTROLLER_MAX) - 1
 } CGroupMask;