ukernel: add morepackets check in listener

ukernel: add morepackets check in listener

Make the listener loop actually check for more packets needed,
like kernel, pthreads and lwp. Only checking for new packets every
20 seconds isn't sufficient on today's networks!

Change-Id: I80319cfd9d540c9a56b0dd8d948a380665426409
Reviewed-on: http://gerrit.openafs.org/5605
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>