Import NetBSD shell environment files

With the import of Xorg, a proper separation between login-time and sub
shell-time of the environment setup is necessary.

Instead of re-developping this from scratch, I am taking the opportunity
to import the NetBSD default environment.

Change-Id: Ib6a8fbd9c2f407ccd59be57a52ef9df21c2c9ce7
VM,MFS: better handling of some exceptional cases

Fix for problems reported by Alejandro Hernández:
	. VM unmap: handle case where there is no nextvr

Fixes for problems found by running Melkor ELF fuzzing tool:
	. VM: better handle case where region prealloc fails by
	  freeing memory that was allocated so far
	. MFS fs_readwrite: EOF check should happen for read and
	  peek requests, not just read

This fixes #4.

Change-Id: I2adf4eebdfb4c48a297beff0478eed5c917a53a4