- Rework the underlying ALQ storage to be a circular buffer, which am…

…ongst other

  things allows variable length messages to be easily supported.

- Extend KPI with alq_writen() and alq_getn() to support variable length
  messages, which is enabled at ALQ creation time depending on the
  arguments passed to alq_open(). Also add variants of alq_open() and
  alq_post() that accept a flags argument. The KPI is still fully
  backwards compatible and shouldn't require any change in ALQ consumers
  unless they wish to utilise the new features.

- Introduce the ALQ_NOACTIVATE and ALQ_ORDERED flags to allow ALQ consumers
  to have more control over IO scheduling and resource acquisition
  respectively.

- Strengthen invariants checking.

- Document ALQ changes in ALQ(9) man page.

Sponsored by:	FreeBSD Foundation
Reviewed by:	gnn, jeff, rpaulo, rwatson
MFC after:	1 month