Add roken/rename.c to fix non-standard rename()

roken/rename.c is for platforms where the native rename()
implementation does not replace the target if it already exists.  This
implementation isn't atomic, but should be close enough for most
purposes.

For correct behavior, rk_rename() should be used instead of rename().
rk_rename() is #defined to be rename() on platforms where this fix is
not necessary.

lib/roken/NTMakefile

@@ -75,6 +75,7 @@ libroken_la_OBJS =			\
 	$(OBJ)\parse_time.obj		\
 	$(OBJ)\parse_units.obj		\
 	$(OBJ)\realloc.obj		\
+	$(OBJ)\rename.obj		\
 	$(OBJ)\resolve.obj		\
 	$(OBJ)\roken_gethostby.obj	\
 	$(OBJ)\rtbl.obj			\