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/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 3dccb22..96779d2 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -170,6 +170,9 @@
 gsm0808_create_sapi_reject;
 gsm0808_create_handover_required;
 gsm0808_create_handover_request_ack;
+gsm0808_create_handover_detect;
+gsm0808_create_handover_complete;
+gsm0808_create_handover_failure;
 gsm0808_prepend_dtap_header;
 gsm0808_enc_aoip_trasp_addr;
 gsm0808_dec_aoip_trasp_addr;