When writing out updated pollfd records when returning from

When writing out updated pollfd records when returning from
poll(), only copy out the revents field, not the whole pollfd
structure.  Otherwise, if the events field is updated
concurrently by another thread, that update may be lost.

This issue apparently causes problems for the JDK on FreeBSD,
which expects the Linux behavior of not updating all fields
(somewhat oddly, Solaris does not implement the required
behavior, but presumably our adaptation of the JDK is based
on the Linux port?).

MFC after:      2 weeks
PR:		kern/130924
Submitted by:   Kurt Miller <kurt @ intricatesoftware.com>
Discussed with:	kib