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/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 755564d..e8433a1 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -154,7 +154,8 @@
 #define DLCSN1		-26	/*!< CSN.1 (Concrete Syntax Notation 1) codec */
 #define DLM2PA		-27	/*!< Osmocom M2PA (libosmo-sigtran) */
 #define DLM2UA		-28	/*!< Reserved for future Osmocom M2UA (libosmo-sigtran) */
-#define OSMO_NUM_DLIB	28	/*!< Number of logging sub-systems in libraries */
+#define DLIO		-29	/*!< Osmocom IO sub-system */
+#define OSMO_NUM_DLIB	29	/*!< Number of logging sub-systems in libraries */
 
 /* Colors that can be used in log_info_cat.color */
 #define OSMO_LOGCOLOR_NORMAL NULL