service: place control command in subcgroup control/

Previously, we were brutally and onconditionally killing all processes
in a service's cgroup before starting the service anew, in order to
ensure that StartPre lines cannot be misused to spawn long-running
processes.

On logind-less systems this has the effect that restarting sshd
necessarily calls all active ssh sessions, which is usually not
desirable.

With this patch control processes for a service are placed in a
sub-cgroup called "control/". When starting a service anew we simply
kill this cgroup, but not the main cgroup, in order to avoid killing any
long-running non-control processes from previous runs.

https://bugzilla.redhat.com/show_bug.cgi?id=805942

src/core/execute.h

@@ -192,6 +192,7 @@ int exec_spawn(ExecCommand *command,
                bool confirm_spawn,
                struct CGroupBonding *cgroup_bondings,
                struct CGroupAttribute *cgroup_attributes,
+               const char *cgroup_suffix,
                pid_t *ret);
 
 void exec_command_done(ExecCommand *c);