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/Makefile.am b/include/osmocom/core/Makefile.am
index e1cd92a..1d394cd 100644
--- a/include/osmocom/core/Makefile.am
+++ b/include/osmocom/core/Makefile.am
@@ -37,6 +37,7 @@
 	msgb.h \
 	netdev.h \
 	netns.h \
+	osmo_io.h \
 	panic.h \
 	prbs.h \
 	prim.h \