osmux: Allocate struct osmux_out_handle through API

Until now, the osmux_out_handle was allocated by the client, and passed
to the API to initialize it. This makes it really hard to improve the
implementation without breaking the ABI.

Let's break the ABI now one last time (hopefully) by allocating the
struct through an API. With only this change, the already built users
(osmo-mgw, openbsc) can still work fine, since there's no change on the
struct osmux_out_handle. However, they will somehow break next time the
struct is changed until they are ported to the same API (easy to do).

Related: OS#5987
Change-Id: Ie8df581f375c9a183a7af60b431561bda82f6e34
---
M TODO-RELEASE
M examples/osmux-test-output.c
M include/osmocom/netif/osmux.h
M src/osmux.c
M tests/jibuf/jibuf_tool.c
M tests/osmux/osmux_test.c
M tests/osmux/osmux_test2.c
7 files changed, 145 insertions(+), 72 deletions(-)