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/Makefile.am b/src/core/Makefile.am
index 50c39d1..80ee458 100644
--- a/src/core/Makefile.am
+++ b/src/core/Makefile.am
@@ -50,6 +50,8 @@
 	msgb.c \
 	netdev.c \
 	netns.c \
+	osmo_io.c \
+	osmo_io_poll.c \
 	panic.c \
 	prbs.c \
 	prim.c \
@@ -107,6 +109,7 @@
 	conv_acc_sse_impl.h \
 	conv_acc_neon_impl.h \
 	crcXXgen.c.tpl \
+	osmo_io_internal.h \
 	stat_item_internal.h \
 	libosmocore.map \
 	$(NULL)