ukernel: set close-on-exec on our socket

ukernel: set close-on-exec on our socket

if the process we are in execs a child, it doesn't get our socket

Reviewed-on: http://gerrit.openafs.org/5598
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 87dcfc872c11fdb13ebf0448abe91150aaa86859)

Change-Id: I7e94e5192783c57aa77afc1b583cccda77ec7653
Reviewed-on: http://gerrit.openafs.org/5704
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>