[PATCH] Add SAP reader stub to libosmosim

Hello.

After overview of libosmosim presented at OsmoDevCon I thought that it would be nice to add SAP backend (implemented by Nico) so it can be transparently integrated with osmocom-bb.

Attached is incomplete attempt to do so. Could you please review code and style?

Also I'm puzzled by write_queue_* functions - should I use them as well or just write to fd directly? In former case - what should I use as event instead of BSC_FD_READ?

In more general - is there some write_queue related description on the wiki?

This patch is obviously not ready to be merged but I hope it might help to speed up development :)