vty: add 'msisdn' as alias for 'extension'

Rationale: in the HLR, it is called 'msisdn' after the database column, so a
user going back and forth between osmo-hlr and osmo-msc would appreciate being
able to type 'msisdn' in the MSC's vty as well.

Change-Id: I7b46f9736421e8edd8a95ae89e025ebe486fde4c
diff --git a/src/libmsc/vty_interface_layer3.c b/src/libmsc/vty_interface_layer3.c
index b0243c3..6def359 100644
--- a/src/libmsc/vty_interface_layer3.c
+++ b/src/libmsc/vty_interface_layer3.c
@@ -343,9 +343,10 @@
 
 	return NULL;
 }
-#define SUBSCR_TYPES "(extension|imsi|tmsi|id)"
+#define SUBSCR_TYPES "(msisdn|extension|imsi|tmsi|id)"
 #define SUBSCR_HELP "Operations on a Subscriber\n"			\
-	"Identify subscriber by extension (phone number)\n"		\
+	"Identify subscriber by MSISDN (phone number)\n"		\
+	"Legacy alias for 'msisdn'\n"		\
 	"Identify subscriber by IMSI\n"					\
 	"Identify subscriber by TMSI\n"					\
 	"Identify subscriber by database ID\n"				\