misc: Add an option exit/quit when the BSSGP is supposed to be destroyed

The PCU does not properly re-set the state when the connection to the
BTS is lost (and the SGSN potentially is re-started during that). This
results in the BSSGP BVCI > 1 remaining blocked and no data will be
accepted by the SGSN.

Add the '-e' option and exit the PCU when the BSSGP/NS are getting
destroyed.
diff --git a/src/pcu_main.cpp b/src/pcu_main.cpp
index 041831f6..754043f 100644
--- a/src/pcu_main.cpp
+++ b/src/pcu_main.cpp
@@ -60,6 +60,7 @@
 			"provided by BTS\n"
 		"  -r   --realtime PRIO Use SCHED_RR with the specified "
 			"priority\n"
+		"  -e   --exit		Exit the application on disconnect\n"
 		);
 }
 
@@ -75,10 +76,11 @@
 			{ "mnc", 1, 0, 'n' },
 			{ "version", 0, 0, 'V' },
 			{ "realtime", 1, 0, 'r' },
+			{ "exit", 0, 0, 'e' },
 			{ 0, 0, 0, 0 }
 		};
 
-		c = getopt_long(argc, argv, "hc:m:n:Vr:",
+		c = getopt_long(argc, argv, "hc:m:n:Vr:e",
 				long_options, &option_idx);
 		if (c == -1)
 			break;
@@ -105,6 +107,9 @@
 		case 'r':
 			rt_prio = atoi(optarg);
 			break;
+		case 'e':
+			gprs_bssgp_exit_on_destroy();
+			break;
 		default:
 			fprintf(stderr, "Unknown option '%c'\n", c);
 			exit(0);