osmo_io: sendmsg/recvmsg support

Add support osmo_io operations resembling sendmsg() and recvmsg() socket
operations.  This is what will enable the implementation of higher-layer
functions like equivalents of sctp_recvmsg() and sctp_send() in
libosmo-netif and/or other users.

Change-Id: I89eb519b22d21011d61a7855b2364bc3c295df82
Related: OS#5751
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index b66e37d..a50a9ed 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -266,7 +266,10 @@
 osmo_iofd_ops;
 osmo_iofd_register;
 osmo_iofd_sendto_msgb;
+osmo_iofd_sctp_send_msgb;
+osmo_iofd_sendmsg_msgb;
 osmo_iofd_set_alloc_info;
+osmo_iofd_set_cmsg_size;
 osmo_iofd_set_data;
 osmo_iofd_set_ioops;
 osmo_iofd_set_priv_nr;