osmux: Use new osmux APIs to let libosmo-netif alloc struct osmux_out_handle

This way we don't need to worry about implementation details (struct
size) in the future in case they change (they will).

Related: OS#5987
Requires: libosmo-netif.git Change-Id Ie8df581f375c9a183a7af60b431561bda82f6e34
Change-Id: I2d0d8c152b8f1234ddfcd74d6cb04b1818b41510
3 files changed