Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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

Modify the alq(9) alq_open() API to accept a file creation mode, rather
than defaulting the cmode argument to vn_open() to 0. Supply a default
argument of ALQ_DEFAULT_CMODE (0600) in current callers.

Discussed with/pointed out by: hmp
Reveiwed by: jeff, hmp
MFC after: 3 days

Remove stale information in IMPLEMENTATION NOTES section about alq_open()
using the calling thread's ucred.

While I am there, fix references to my name.

Note that functions alq_flush() and alq_post() should not be called in
contexts where sleeping is disallowed.

Pointed out by: ru

Bring the prototype for alq_open() in line with the code.

Add a section on locking.

Sweep formatting and assorted fixes.

Manual page for Asynchronous Logging Queues (ALQ) facility.

The manual page contains enough information to get someone started
with ALQ.

MLINKS have been added appropriately.

Approved by: jeff, des
Reviewed by: des, jeff, sam, brooks, rwatson, mtm