Add osmo_io with initial poll backend

* make backend configurable for later
* segmentation callback for chunked streams
* logging target for osmo_io
* support partial writes

Change-Id: I50d73cf550d6ce8154bf827bf47408131cf5b0a0
Related: SYS#5094, OS#5751
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index 095a4c0..7bca331 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -252,6 +252,30 @@
 osmo_init_logging2;
 osmo_int_to_float_str_buf;
 osmo_int_to_float_str_c;
+osmo_io_backend_names;
+osmo_iofd_close;
+osmo_iofd_free;
+osmo_iofd_get_data;
+osmo_iofd_get_fd;
+osmo_iofd_get_name;
+osmo_iofd_get_priv_nr;
+osmo_iofd_init;
+osmo_iofd_ops;
+osmo_iofd_read_disable;
+osmo_iofd_read_enable;
+osmo_iofd_register;
+osmo_iofd_sendto_msgb;
+osmo_iofd_set_alloc_info;
+osmo_iofd_set_data;
+osmo_iofd_set_priv_nr;
+osmo_iofd_setup;
+osmo_iofd_txqueue_clear;
+osmo_iofd_txqueue_len;
+osmo_iofd_unregister;
+osmo_iofd_uring_init;
+osmo_iofd_write_disable;
+osmo_iofd_write_enable;
+osmo_iofd_write_msgb;
 osmo_ip_str_type;
 osmo_isdnhdlc_decode;
 osmo_isdnhdlc_encode;