PAX ASLR: fix mmap randomization which breaked by 3d154ea876554af90f920eb85de6cb733dc5f267 commit

After 3d154ea876554af90f920eb85de6cb733dc5f267 commit the paxtest's
output are this:

PaXtest - Copyright(c) 2003,2004 by Peter Busser <peter@adamantix.org>
Released under the GNU Public Licence version 2 or later

Mode: kiddie
FreeBSD  11.0-CURRENT FreeBSD 11.0-CURRENT #3
d6b9ed4(op/hardened/current/aslr-devel)-dirty: Fri Aug  8 17:06:55
CEST 2014     op@pandora-d:/tmp/amd64-objdir/usr/data/source/git/opBSD/opBSD.git/sys/GENERIC
 amd64

Executable anonymous mapping             : Killed
Executable bss                           : Killed
Executable data                          : Killed
Executable heap                          : Killed
Executable stack                         : Killed
Executable anonymous mapping (mprotect)  : Vulnerable
Executable bss (mprotect)                : Vulnerable
Executable data (mprotect)               : Vulnerable
Executable heap (mprotect)               : Vulnerable
Executable shared library bss (mprotect) : Vulnerable
Executable shared library data (mprotect): Vulnerable
Executable stack (mprotect)              : Vulnerable
Anonymous mapping randomisation test     : No randomisation
Heap randomisation test (ET_EXEC)        : 2 bits (guessed)
Heap randomisation test (PIE)            : 12 bits (guessed)
Main executable randomisation (ET_EXEC)  : No randomisation
Main executable randomisation (PIE)      : 21 bits (guessed)
Shared library randomisation test        : No randomisation
Stack randomisation test (SEGMEXEC)      : 15 bits (guessed)
Stack randomisation test (PAGEEXEC)      : 16 bits (guessed)
Arg/env randomisation test (SEGMEXEC)    : 17 bits (guessed)
Arg/env randomisation test (PAGEEXEC)    : 17 bits (guessed)
Randomization under memory exhaustion @~0: No randomisation
Randomization under memory exhaustion @0 : No randomisation
Return to function (strcpy)              : paxtest: return address
contains a NULL byte.
Return to function (strcpy, PIE)         : paxtest: return address
contains a NULL byte.
Return to function (memcpy)              : Killed
Return to function (memcpy, PIE)         : Killed
Executable shared library bss            : Killed
Executable shared library data           : Killed
Writable text segments                   : Vulnerable

After partially revert the mentioned commit, the paxtest's output fixed:

FreeBSD pandora-d 10.0-STABLE FreeBSD 10.0-STABLE #72
6fd52c6(op/stable/10/opbsd-aslr): Sat Aug  9 02:33:44 CEST 2014     root@pandora-d:/usr/obj/usr/src/sys/OP-ASLR  amd64

Executable anonymous mapping             : Killed
Executable bss                           : Killed
Executable data                          : Killed
Executable heap                          : Killed
Executable stack                         : Killed
Executable anonymous mapping (mprotect)  : Vulnerable
Executable bss (mprotect)                : Vulnerable
Executable data (mprotect)               : Vulnerable
Executable heap (mprotect)               : Vulnerable
Executable shared library bss (mprotect) : Vulnerable
Executable shared library data (mprotect): Vulnerable
Executable stack (mprotect)              : Vulnerable
Anonymous mapping randomisation test     : 21 bits (guessed)
Heap randomisation test (ET_EXEC)        : 11 bits (guessed)
Heap randomisation test (PIE)            : 12 bits (guessed)
Main executable randomisation (ET_EXEC)  : No randomisation
Main executable randomisation (PIE)      : 21 bits (guessed)
Shared library randomisation test        : 21 bits (guessed)
Stack randomisation test (SEGMEXEC)      : 15 bits (guessed)
Stack randomisation test (PAGEEXEC)      : 16 bits (guessed)
Arg/env randomisation test (SEGMEXEC)    : 17 bits (guessed)
Arg/env randomisation test (PAGEEXEC)    : 16 bits (guessed)
Randomization under memory exhaustion @~0: 22 bits (guessed)
Randomization under memory exhaustion @0 : 21 bits (guessed)
Return to function (strcpy)              : paxtest: return address contains a NULL byte.
Return to function (strcpy, PIE)         : paxtest: return address contains a NULL byte.
Return to function (memcpy)              : Killed
Return to function (memcpy, PIE)         : Killed
Executable shared library bss            : Killed
Executable shared library data           : Killed
Writable text segments                   : Vulnerable

Signed-off-by: Oliver Pinter <oliver.pntr@gmail.com>
PAX ASLR: move Shawn's hack to better place

Signed-off-by: Oliver Pinter <oliver.pntr@gmail.com>
PAX ASLR: introduce security.pax.aslr_allow_MAP_32BIT tunable

Signed-off-by: Oliver Pinter <oliver.pntr@gmail.com>