jaildesc: add kevent support

Give jail descriptors the same kevent flags as jails.  Also fix the
event reporting in jails, where it was including data for events the
user didn't ask for.

MFC after:	3 days

lib/libsys/kqueue.2

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 11, 2025
+.Dd September 12, 2025
 .Dt KQUEUE 2
 .Os
 .Sh NAME
@@ -638,6 +638,19 @@ or
 .Dv NOTE_JAIL_CHILD
 event has been received since the last call to
 .Fn kevent .
+.It Dv EVFILT_JAILDESC
+Takes a jail descriptor returned by
+.Xr jail_set 2
+or
+.Xr jail_get 2
+as the identifier and the events to watch for in
+.Va fflags ,
+and returns when the jail performs one or more of the requested events.
+The events to monitor and the resulting
+.Va fflags
+are the same as those listed in
+.Dv EVFILT_JAIL ,
+above.
 .It Dv EVFILT_TIMER
 Establishes an arbitrary timer identified by
 .Va ident .