Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

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)