osmo_io: Add osmo_iofd_notify_connected()

Don't call write_enable() in osmo_iofd_register(). This was used to
detect whether a socket is connected or not, but would always be
enabled, even on unconnected sockets. Instead make this behaviour
explicit by calling osmo_iofd_notify_connected().

Change-Id: Ieed10bc94c8aad821c0a8f7764db0e05c054c1e3
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index d31392b..50be67c 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -273,6 +273,7 @@
 osmo_iofd_txqueue_len;
 osmo_iofd_unregister;
 osmo_iofd_uring_init;
+osmo_iofd_notify_connected;
 osmo_iofd_write_msgb;
 osmo_ip_str_type;
 osmo_isdnhdlc_decode;