add and tweak inter-BSC HO API

Add:
gsm0808_create_handover_detect()
gsm0808_create_handover_complete()
gsm0808_create_handover_failure()

To existing structs gsm0808_old_bss_to_new_bss_info and
gsm0808_handover_required, add a final 'more_items' flag that makes future
extensions API and ABI compatible.

Fix the msgb string for Handover Request Ack.

Extend some API doc comments.

Related: OS#2283 (inter-BSC Handover, BSC side, MT)
Change-Id: I03ee7ce840ecfa0b6a33358e7385528aabd4873f
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 83e29ca..6ff4459 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -30,6 +30,8 @@
 
 #include <osmocom/core/defs.h>
 #include <osmocom/core/utils.h>
+#include <osmocom/gsm/protocol/gsm_08_08.h>
+#include <osmocom/gsm/protocol/gsm_04_08.h>
 
 #define ADD_MODULO(sum, delta, modulo) do {	\
 	if ((sum += delta) >= modulo)		\