queue(3): New *_SPLIT_AFTER(), *_ASSERT_EMPTY(), *_ASSERT_NONEMPTY()

*_SPLIT_AFTER() allows to split an existing queue in two. It is the
missing block that enables arbitrary splitting and recombinations of
lists/queues together with *_CONCAT() and *_SWAP().

Add *_ASSERT_NONEMPTY(), used by *_SPLIT_AFTER().

Reviewed by: markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49608 (stailq)
Differential Revision: https://reviews.freebsd.org/D49969 (rest)