abis/close: Deliver S_L_INP_TEI_DN when closing the socket

* HSL/IPA had different socket closing code for the same thing,
  create one method for it.
* Both methods tried to send an event but as we are on the close
  path the sign_link was already removed from the list and the
  input event sending method couldn't find the sign_link using the
  sapi/tei provided.
diff --git a/include/internal.h b/include/internal.h
index 6969748..ddb2ae2 100644
--- a/include/internal.h
+++ b/include/internal.h
@@ -3,6 +3,10 @@
 
 #include <stdint.h>
 
+struct osmo_fd;
+struct e1inp_sign_link;
+struct e1inp_ts;
+
 /* talloc context for libosmo-abis. */
 extern void *libosmo_abis_ctx;
 
@@ -15,4 +19,13 @@
 struct msgb *ipa_msg_alloc(int headroom);
 void ipa_msg_push_header(struct msgb *msg, uint8_t proto);
 
+/*
+ * helper for internal drivers, not public
+ */
+void e1inp_close_socket(struct e1inp_ts *ts,
+			struct e1inp_sign_link *sign_link,
+			struct osmo_fd *bfd);
+
+
+
 #endif