oxenstored: force FD_CLOEXEC with Unix.set_close_on_exec on LSB init

oxenstored: force FD_CLOEXEC with Unix.set_close_on_exec on LSB init

Lets match the systemd active socket activation implementation and
ensure that FD_CLOEXEC is set by usin Unix.set_close_on_exec. David
notes oxenstored likely does not exec but there is no harm in being
careful just in case things change in the future.

Cc: David Scott <dave.scott@eu.citrix.com>
Cc: Anil Madhavapeddy <anil@recoil.org>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Vincent Hanquez <Vincent.Hanquez@eu.citrix.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>