lib/tevent - fix processing of messaging idle timer

PR: https://github.com/freenas/ports/pull/909

The pluggable backend for libtevent lacks a entry for update_timer
in tevent_ops(). This function from lib/tevent/tevent_timed.c is
called in messaging_dgm_out_rearm_idle_timer in s3/lib/messages_dgm.c.

This implements minimal fix to replace the standard timer_update()
function with a call to update the existing kevent. Future fix will expand tevent_ops()
to include an update_timer operation.