core: port unit_main_pid() + unit_control_pid() to PidRef and drop unit_kill_common()

This ports over unit_main_pid() + unit_control_pid() to return PidRef*
pointers (which also means the underlying UnitVTable function pointers
are changed accordingly).

This then uses te functions to simplify the unit_kill() call, by
avoiding the kill() vtable indirection and instead just suing
unit_main_pid() and unit_control_pid() directly.

src/core/slice.c

@@ -247,10 +247,6 @@ static int slice_stop(Unit *u) {
         return 1;
 }
 
-static int slice_kill(Unit *u, KillWho who, int signo, int code, int value, sd_bus_error *error) {
-        return unit_kill_common(u, who, signo, code, value, -1, -1, error);
-}
-
 static int slice_serialize(Unit *u, FILE *f, FDSet *fds) {
         Slice *s = SLICE(u);
 
@@ -436,8 +432,6 @@ const UnitVTable slice_vtable = {
         .start = slice_start,
         .stop = slice_stop,
 
-        .kill = slice_kill,
-
         .freeze = slice_freeze,
         .thaw = slice_thaw,
         .can_freeze = slice_can_freeze,