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/openbts_sock.cpp b/src/openbts_sock.cpp
index 845aa77..a09f834 100644
--- a/src/openbts_sock.cpp
+++ b/src/openbts_sock.cpp
@@ -179,7 +179,7 @@
 
 void pcu_l1if_close(void)
 {
-	gprs_bssgp_destroy();
+	gprs_bssgp_destroy_or_exit();
 
 	/* FIXME: cleanup l1if */
 	talloc_free(l1fh->fl1h);