Use XRDP_SOCKET_PATH in file_loc.h
Don't define any non-socket paths in file_loc.h, they should come from
the makefiles.
Define all paths unconditionally, they should not be defined elsewhere.
Pass XRDP_SOCKET_PATH as environment variable to the backends.
libdir is for shared and static libraries. Some xrdp libraries are loaded
dynamically from a non-standard directory, so they are used like modules.
Having separate libdir and moduledir would eventually allow to separate
dynamically loaded modules from shared libraries.