systemd: Do not restart with a broken config file or such

Only restart in case of a crash or the exit(42) when the OML/RSL
link is going down.

contrib/sysmobts.service

@@ -9,6 +9,7 @@ ExecStopPost=/bin/sh -c 'echo 0 > /sys/class/leds/activity_led/brightness'
 ExecStopPost=/bin/sh -c 'cat /lib/firmware/sysmobts-v?.bit > /dev/fpgadl_par0 ; sleep 3s; cat /lib/firmware/sysmobts-v?.out > /dev/dspdl_dm644x_0; sleep 1s'
 Restart=always
 RestartSec=2
+RestartPreventExitStatus=1
 
 # The msg queues must be read fast enough
 CPUSchedulingPolicy=rr

src/osmo-bts-sysmo/main.c

@@ -351,7 +351,7 @@ int main(int argc, char **argv)
 
 	if (pcu_sock_init()) {
 		fprintf(stderr, "PCU L1 socket failed\n");
-		exit(-1);
+		exit(1);
 	}
 
 	signal(SIGINT, &signal_handler);