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)

I completely fubared this. An empty EXPORT_SYMS= is not valid. I know I
build this several times, and even tested loaded the module. I must have
changed something right before commit.

Pointed out by: bde

opt_kstack_pages.h is not needed anymore. It would have been a Bad Thing
if it had been different to the running kernel.

Add an a.out exec module.