make the message about null page less OS specific (suggest linux
sysctl as well)


(maya)

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$NetBSD: MESSAGE,v 1.3 2016/12/26 22:36:30 maya Exp $

If you are experiencing crashes, it may be due to a security measure
blocking the mapping of the NULL page.

You can disable this (at a security cost) with the following commands:

 NetBSD:
	# sysctl -w vm.user_va0_disable=0
 Linux:
	# sysctl -w vm.mmap_min_addr=0

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