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/shared/bus-util.c

@@ -1421,7 +1421,7 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                 return bus_log_create_error(r);
 
         if (STR_IN_SET(field,
-                       "CPUAccounting", "MemoryAccounting", "BlockIOAccounting",
+                       "CPUAccounting", "MemoryAccounting", "BlockIOAccounting", "TasksAccounting",
                        "SendSIGHUP", "SendSIGKILL", "WakeSystem", "DefaultDependencies",
                        "IgnoreSIGPIPE", "TTYVHangup", "TTYReset", "RemainAfterExit")) {
 
@@ -1436,14 +1436,33 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
         } else if (streq(field, "MemoryLimit")) {
                 off_t bytes;
 
-                r = parse_size(eq, 1024, &bytes);
-                if (r < 0) {
-                        log_error("Failed to parse bytes specification %s", assignment);
-                        return -EINVAL;
+                if (isempty(eq) || streq(eq, "infinity"))
+                        bytes = (uint64_t) -1;
+                else {
+                        r = parse_size(eq, 1024, &bytes);
+                        if (r < 0) {
+                                log_error("Failed to parse bytes specification %s", assignment);
+                                return -EINVAL;
+                        }
                 }
 
                 r = sd_bus_message_append(m, "v", "t", (uint64_t) bytes);
 
+        } else if (streq(field, "TasksMax")) {
+                uint64_t n;
+
+                if (isempty(eq) || streq(eq, "infinity"))
+                        n = (uint64_t) -1;
+                else {
+                        r = safe_atou64(eq, &n);
+                        if (r < 0) {
+                                log_error("Failed to parse maximum tasks specification %s", assignment);
+                                return -EINVAL;
+                        }
+                }
+
+                r = sd_bus_message_append(m, "v", "t", n);
+
         } else if (STR_IN_SET(field, "CPUShares", "BlockIOWeight")) {
                 uint64_t u;