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/TODO-RELEASE b/TODO-RELEASE
index bade6f5..7270257 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -10,3 +10,4 @@
 libosmogsm  new header osmocom/gsm/protocol/gsm_44_060.h
 libosmocore ADD     new defines in osmocom/gsm/protocol/gsm_04_08.h (old ones marked deprecated)
 libosmovty	drop API		drop struct vty_parent_node from public API, it should never have been public
+libosmocore ADD     new API osmo_io_*()