Add the "pg update" svc action

Which applies to cgroups changed values of pg_* keywords.

$ om test/svc/c1 pg update
$ om test/svc/c1 set --kw app#0.pg_mem_limit=100m
$ om test/svc/c1 pg update
@ n:aubergine o:test/svc/c1 r:app#0 sc:n
  /bin/echo 104857600 > /sys/fs/cgroup/memory/opensvc.slice/test.slice/c1.slice/app.slice/app.0.slice/memory.limit_in_bytes

opensvc/commands/svc/parser.py

@@ -576,6 +576,10 @@
         "msg": "Kill the tasks of a process group.",
         "options": mp.ACTION_OPTS,
     },
+    "pg_update": {
+        "msg": "Update cappings of process groups to reflect configuration changes.",
+        "options": mp.ACTION_OPTS,
+    },
 }
 ACTIONS.update({
     "Compliance": {

opensvc/core/objects/svc.py

@@ -364,6 +364,7 @@ def signal_handler(*args):
 ACTIONS_DO_MASTER_AND_SLAVE = [
     "boot",
     "migrate",
+    "pg_update",
     "provision",
     "prstart",
     "prstop",
@@ -3921,6 +3922,9 @@ def pg_remove(self):
     def pg_pids(self):
         return sorted(self.pg.pids(self))
 
+    def pg_update(self):
+        self.sub_set_action(START_GROUPS, "create_pg", xtags=set(["zone", "docker", "podman"]))
+
     @lazy
     def pg(self):
         """