Add a new jail OSD method, PR_METHOD_REMOVE.  It's called when a jail is
removed from the user perspective, i.e. when the last pr_uref goes away,
even though the jail mail still exist in the dying state.  It will also
be called if either PR_METHOD_CREATE or PR_METHOD_SET fail.

Delay removing the last jail reference in prison_proc_free, and instead
put it off into the pr_task.  This is similar to prison_free, and in fact
uses the same task even though they do something slightly different.

Pass the current/new jail to PR_METHOD_CHECK, which pushes the call
until after the jail is found or created.  This requires unlocking the
jail for the call and re-locking it afterward, but that works because
nothing in the jail has been changed yet, and other processes won't
change the important fields as long as allprison_lock remains held.

Keep better track of name vs namelc in kern_jail_set.  Name should
always be the hierarchical name (relative to the caller), and namelc
the last component.

Use crcopysafe in jail_attach.