ipa driver: make bind address vty configurable

Add VTY function to set the ipa bind address:
    e1_input
     ipa bind A.B.C.D

Add a priv pointer to struct e1inp_driver in order to communicate the bind
address parameter to ipaccess_line_update(). Add two "internal.h" functions to
get/set it in the ipa driver struct.

Add static ip_bind_addr() to use the IP address set from the VTY or, if NULL,
use "0.0.0.0". Apply in ipaccess_line_update().
diff --git a/include/internal.h b/include/internal.h
index 7f6e31a..c931d4f 100644
--- a/include/internal.h
+++ b/include/internal.h
@@ -13,6 +13,9 @@
 /* use libosmo_abis_init, this is only for internal use. */
 void e1inp_init(void);
 
+void e1inp_ipa_set_bind_addr(const char *ip_bind_addr);
+const char *e1inp_ipa_get_bind_addr(void);
+
 /* ipaccess.c requires these functions defined here */
 struct msgb;
 struct msgb *ipa_msg_alloc(int headroom);