Deferred freeing of kauth_cred_ts

[Elad Efrat <elad%NetBSD.org@localhost>]

Hi,

One thing that prevents us from attaching credentials to sockets is an
issue where these might be freed from interrupt context.

What I have in mind is being able to tell when we can sleep or not,
and if not, add the credentials to a queue that an independent thread
will take care of freeing.

If this is sensible, my first step will be extracting the guts of
assert_sleepable() to is_sleepable() so we don't always trip a panic()
if we can't sleep.

Thoughts?

Thanks,

-e.