Nightly build from 2021-11-13-02-00-26

Split directory.cpp into unix/win32 parts (#491)

directory.cpp was essentially two implementations in one, separated with ifdefs. It's cleaner to just have two separate files, seeing how CMakeLists.txt for libmediation needs to check the platform it's running on and add the appropriate file implementation anyway.

This commit splits this file into two and extracts common code parts to directory_priv.h.