systemctl: add env var to force connection to system manager via the bus

Sometimes it is useful for debugging purposes to force systemctl to connect to
PID 1 via the bus instead of direct connection, even if the direct connection
is possible.

src/systemctl/systemctl.c

@@ -204,6 +204,9 @@ static int acquire_bus(BusFocus focus, sd_bus **ret) {
         if (arg_transport != BUS_TRANSPORT_LOCAL)
                 focus = BUS_FULL;
 
+        if (getenv_bool("SYSTEMCTL_FORCE_BUS") > 0)
+                focus = BUS_FULL;
+
         if (!busses[focus]) {
                 bool user;