Fixes in callback handling when fds are moved between backends. The backend is now set from the fd object instead to allow overriding it in Stdio.File-like classes. Rev: src/backend.cmod:1.58 Rev: src/backend.h:1.18 Rev: src/modules/files/file.c:1.302 Rev: src/modules/files/file_functions.h:1.30 Rev: src/modules/files/socket.c:1.79