sms: Give smsc its own VTY config node

The pre-historic sms_queue code used to have very strange aspects,
such as having some parameters (max-failure, max-pending) which could
only be sent from the 'enable' node, but not from a config file.

Before adding more configuration parameters, let's clean this up by
introducing a proper VTY config node for the 'smsc'; move the existing
config commands there and add new ones for max-failure and max-pending.

As the sms_queue data structure is only allocated after the config file
parsing happens, we are introducing a new 'sms_queue_config' data
structure.  This encapsulates the public readable/writable config
parameters.

Change-Id: Ie8e0ab1a9f979337ff06544b9ab3820954d9804a
diff --git a/src/libmsc/Makefile.am b/src/libmsc/Makefile.am
index e6a2dc1..d3b6035 100644
--- a/src/libmsc/Makefile.am
+++ b/src/libmsc/Makefile.am
@@ -67,6 +67,7 @@
 	sdp_msg.c \
 	silent_call.c \
 	sms_queue.c \
+	smsc_vty.c \
 	transaction.c \
 	msc_net_init.c \
 	ctrl_commands.c \