util-lib: split out fd-related operations into fd-util.[ch]

There are more than enough to deserve their own .c file, hence move them
over.

src/basic/async.c

@@ -23,6 +23,7 @@
 #include <unistd.h>
 
 #include "async.h"
+#include "fd-util.h"
 #include "log.h"
 #include "util.h"