While sysctl_remove_oid() may recusrively try to delete all
sysctl nodes, if one is still referenced it may panic (bd3cdc3105aae).
Use a sysctl_ctx_list to nicely group them all together
and also to release them -- at beast -- with a leak.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

Just putting this out. Still not sure it's the best idea and
fully correct. Done while debugging some other crashes.