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

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

MFC r253526:
Fix several warnings.
Fix crash in aio_pty_cleanup() by initializing the pointer before
dereferencing.

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

Approved by: re (implicit)

Add missing headers.

MFC after: 3 days

Print a warning if running as !root for aio_md_test rather than failing
the test. Privilege is required in order to allocate an md device.

Add a basic aio functionality regression test, which simply writes and
then reads from a fairly broad range of object types: regular file,
fifo, UNIX socketpair, pty, UNIX pipe, and an md device. Not a deep
test of functionality, just a basic test that aio_write followed by
aio_read returns the correct data in a relatively timely manner.

Requested by: phk