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