socket_compat.h improvements to always have struct osmo_sockaddr available
Having a #if (!EMBEDDED) based on compiler defined variable is not a good idea. Instead, add minimal sys/socket.h and netient/in.h to always have the definition of struct osmo_sockaddr available.
remove references to define EMBEDDED in public headers
It's not a good idea to base opt-in/out of code in public headers based on some specific non-standard define being set at compile time when calling the compiler. Get rid of those references; Now that osmocom/core/socket.h can be included thanks to improved osmocom/core/socket_compat.h, it shouldn't be a problem.
We already have all other log_target implementations each on its own file. Move the file (and stderr, a specific case of file) into its own file too, properly separating most file-specific logic from general logging logic.
