ipaccess: initial works to get BTS mode working

This patch adds the initial support to get BTS mode working with
the ipaccess driver.

Now, the driver handles IPA ping, pong and id_ack messages
internally in BTS modes, and it passes the signalling messages
to the client application by invoking the callback line operations.

Moreover, with this patch, each IPA link object always has one
E1 line object associated.

Still HSL BTS-mode remains unimplemented.
diff --git a/src/input/hsl.c b/src/input/hsl.c
index cb1aa51..f8c009e 100644
--- a/src/input/hsl.c
+++ b/src/input/hsl.c
@@ -302,6 +302,12 @@
         return ret;
 }
 
+static int hsl_bts_process(struct ipa_link *link, struct msgb *msg)
+{
+	/* XXX: not implemented yet. */
+	return 0;
+}
+
 static int hsl_line_update(struct e1inp_line *line,
 			   enum e1inp_line_role role, const char *addr)
 {
@@ -331,7 +337,8 @@
 
 		LOGP(DINP, LOGL_NOTICE, "enabling hsl BTS mode\n");
 
-		link = ipa_client_link_create(tall_hsl_ctx, addr, HSL_TCP_PORT);
+		link = ipa_client_link_create(tall_hsl_ctx, line, addr,
+						HSL_TCP_PORT, hsl_bts_process);
 		if (link == NULL) {
 			LOGP(DINP, LOGL_ERROR, "cannot create BTS link: %s\n",
 				strerror(errno));