Refine fix to avoid passing timeout-usecs as #f.

This is a more correct fix than that reverted in the (now reverted) commit
2014b87 ("Do not apply udev-monitor-set-timeout! to #f values") to correctly
handle the #:timeout-usecs keyword argument, which should not be specified
as #f per Guile's 'select' API.

* modules/udev/monitor.scm (make-udev-monitor): Avoid applying
udev-monitor-set-timeout! to a #f 3rd argument (usecs).

modules/udev/monitor.scm

@@ -63,7 +63,11 @@ seconds and microseconds, respectively.  If TIMEOUT-USEC, is used, TIMEOUT-SEC
 must also have a value, else it is ignored, as for the 'secs' and 'usecs'
 argument of Guile's 'select' procedure."
   (let ((monitor (%make-udev-monitor udev)))
-    (udev-monitor-set-timeout!  monitor timeout-sec timeout-usec)
+    ;; Guile's select accepts #f for its optional 'secs' argument,
+    ;; but not for usecs'.
+    (if timeout-usec
+        (udev-monitor-set-timeout! monitor timeout-sec timeout-usec)
+        (udev-monitor-set-timeout! monitor timeout-sec))
     (udev-monitor-set-callback! monitor callback)
     (udev-monitor-set-error-callback! monitor error-callback)
     (when filter