jail: simplify EVFILT_JAIL events

Instead of using the EVFILT_PROC model of attempting to automatically
register new events when a child jail is created, just give a single
event when a child jail is created.  As was already done with jail
attach events, make a best-effort report of the added jail's id in
kn_data.  If the are multiple NOTE_JAIL_CHILD and/or NOTE_JAIL_ATTACH
events, set the NOTE_JAIL_MULTI flag, and don't report anything in
data, indicating that the caller will need to query the system state
on their own.

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 4, 2025
+.Dd September 11, 2025
 .Dt KQUEUE 2
 .Os
 .Sh NAME
@@ -614,41 +614,30 @@ The process ID will be stored in
 If more than one process has attached since the last call to
 .Fn kevent ,
 .Va data
-will contain the most recently attached process ID,
-with
-.Dv NOTE_JAIL_ATTACH_MULTI
-set in
-.Va fflags .
+will be zero.
 .It Dv NOTE_JAIL_REMOVE
 The jail has been removed.
 .It Dv NOTE_JAIL_CHILD
 A child of the watched jail has been created.
-.It Dv NOTE_TRACK
-Follow child jails created under this jail.
-Register a new kevent to monitor the child jail using the same
-.Va fflags
-as the original event.
-The child jail will signal an event with
-.Dv NOTE_CHILD
-set in
-.Va fflags
-and the parent JID in
+Its jail ID will be stored in
 .Va data .
-.Pp
-If registering a new kevent fails
-.Pq usually due to resource limitations ,
-it will signal an event with
-.Dv NOTE_TRACKERR
-set in
-.Va fflags ,
-and the child jail will not signal a
-.Dv NOTE_CHILD
-event.
+If more than one jail has been created since the last call to
+.Fn kevent ,
+.Va data
+will be zero.
 .El
 .Pp
 On return,
 .Va fflags
 contains the events which triggered the filter.
+It will also contain
+.Dv NOTE_JAIL_MULTI
+if more than one
+.Dv NOTE_JAIL_ATTACH
+or
+.Dv NOTE_JAIL_CHILD
+event has been received since the last call to
+.Fn kevent .
 .It Dv EVFILT_TIMER
 Establishes an arbitrary timer identified by
 .Va ident .