move various generic IPA related functions to libosmocore

libosmo-abis is about forming A-bis interfaces/lines by means
of E1 or the IPA multiplex (or possibly other link layers).

The IPA multiplex is used in other contexts, such as the Control
interface, or the A interface.  In that context, it makes sense to have
generic IPA related functions in libosmocore.
diff --git a/tests/e1inp_ipa_bts_test.c b/tests/e1inp_ipa_bts_test.c
index 02a4cb3..18f1709 100644
--- a/tests/e1inp_ipa_bts_test.c
+++ b/tests/e1inp_ipa_bts_test.c
@@ -9,6 +9,7 @@
 #include <osmocom/core/application.h>
 #include <osmocom/abis/ipaccess.h>
 #include <osmocom/gsm/protocol/gsm_12_21.h>
+#include <osmocom/gsm/ipa.h>
 
 static void *tall_test;
 static struct e1inp_sign_link *oml_sign_link, *rsl_sign_link;