FreeBSD_HEAD_arm64 - Build #3662 - Fixed

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FreeBSD_HEAD_arm64 - Build #3662 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/3662/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/3662/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_arm64/3662/console

Change summaries:

303166 by imp:
Add opt_ddb.h.

MFC after: 1 week

303165 by imp:
Fix cam builds w/o nvme. This is a stop-gap fix until the real fix of
dynamically loading XPTs is done and tested.

MFC After: 1 week

303164 by jhb:
Add more documentation regarding unsafe AIO requests.

The asynchronous I/O changes made previously result in different
behavior out of the box. Previously all AIO requests failed with
ENOSYS / SIGSYS unless aio.ko was explicitly loaded. Now, some AIO
requests complete and others ("unsafe" requests) fail with EOPNOTSUPP.

Reword the introductory paragraph in aio(4) to add a general
description of AIO before describing the vfs.aio.enable_unsafe sysctl.

Remove the ENOSYS error description from aio_fsync(2), aio_read(2),
and aio_write(2) and replace it with a description of EOPNOTSUPP.

Remove the ENOSYS error description from aio_mlock(2).

Log a message to the system log the first time a process requests an
"unsafe" AIO request that fails with EOPNOTSUPP. This is modeled on
the log message used for processes using the legacy pty devices.

Reviewed by:	kib (earlier version)
MFC after:	1 week
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D7151