Changes

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.

Change-Id: I5b10e09770727c72812af15360ab3385b957509b

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.

Change-Id: I7a657d67077dfbe861959c6123eb9163bc791694

logging: Move log target file to its own file

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.

Change-Id: I8e32e31c75b66ff0649d92c2f469f8895689fbad