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)

Replace the name of the sysctl to security.bsd.map_at_zero and to be
consistent updated the name of the variable as well, after the change
in r197711.

Add a simple C program to check mmap calls to various different addresses.
The most important test is the mapping fixed at address 0 depending on the
new sysctl.

Things will be updated and possibly converted to m4/.t style once the
details about the kernel patch will be shaken out.

Submitted by: simon (initial version)