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)

Add open_to_operation, a security regression test that opens files with
various open flags and then tests various operations to make sure that
they are properly constrained by open flags. Various I/O mechansms
are tried, including aio if compiled into the kernel or loaded as a
module. There's more to be done here but it's a useful start, running
about 220 individual tests.

This is in support of FreeBSD-SA-08:03.sendfile.