Do not allow mmap with the MAP_FIXED argument to map at address zero.

Do not allow mmap with the MAP_FIXED argument to map at address zero.
This is done to make it harder to exploit kernel NULL pointer security
vulnerabilities.  While this of course does not fix vulnerabilities,
it does mitigate their impact.

Note that this may break some applications, most likely emulators or
similar, which for one reason or another require mapping memory at
zero.

This restriction can be disabled with the security.bsd.mmap_zero
sysctl variable.

Discussed with:	rwatson, bz
Tested by:	bz (Wine), simon (VirtualBox)
Submitted by:	jhb