machined: Allow user ids in open_shell for machine-dbus

Previously "machinectl shell --uid=1000 <container>" resulted in a
sucessful drop into a shell in the respective target machine. After
commit a9e9288288 this is no longer the
case.

This fixes the above breaking change brought in commit a9e9288288

src/machine/machine-dbus.c

@@ -391,7 +391,7 @@ int bus_machine_method_open_shell(sd_bus_message *message, void *userdata, sd_bu
                 return r;
         user = isempty(user) ? "root" : user;
 
-        if (!valid_user_group_name(user, VALID_USER_RELAX))
+        if (!valid_user_group_name(user, VALID_USER_RELAX | VALID_USER_ALLOW_NUMERIC))
                 return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid user name '%s'", user);
 
         /* Ensure only root can shell into the root namespace. This is to avoid unprivileged users registering