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/logging.c b/src/core/logging.c
index 3095f0d..c6774f5 100644
--- a/src/core/logging.c
+++ b/src/core/logging.c
@@ -313,6 +313,12 @@
 		.enabled = 1, .loglevel = LOGL_NOTICE,
 		.color = "\033[38;5;11m",
 	},
+	[INT2IDX(DLIO)] = {
+		.name = "DLIO",
+		.description = "libosmocore IO Subsystem",
+		.enabled = 1, .loglevel = LOGL_NOTICE,
+		.color = "\033[38;5;67m",
+	},
 };
 
 void assert_loginfo(const char *src)