Modified task initialisation so that stack now comes beneath the environment

Environment is backed by a special per-task file maintained by mm0 for each task.
This file is filled in by the env pager, by simple copying of env data into the
faulty page upon a fault. UTCB and all anon regions (stack) could use the same
scheme.

Fixed IS_ERR(x) to accept negative values that are above -1000 for errors. This
protects against false positives for pointers such as 0xE0000000.

	modified:   include/l4/generic/scheduler.h
	modified:   include/l4/macros.h
	modified:   src/arch/arm/exception.c
	modified:   tasks/fs0/include/linker.lds
	modified:   tasks/libl4/src/init.c
	modified:   tasks/libposix/shm.c
	new file:   tasks/mm0/include/env.h
	modified:   tasks/mm0/include/file.h
	new file:   tasks/mm0/include/lib/addr.h
	deleted:    tasks/mm0/include/lib/vaddr.h
	modified:   tasks/mm0/include/task.h
	new file:   tasks/mm0/include/utcb.h
	new file:   tasks/mm0/src/env.c
	modified:   tasks/mm0/src/fault.c
	modified:   tasks/mm0/src/file.c
	modified:   tasks/mm0/src/init.c
	new file:   tasks/mm0/src/lib/addr.c
	modified:   tasks/mm0/src/lib/idpool.c
	deleted:    tasks/mm0/src/lib/vaddr.c
	modified:   tasks/mm0/src/mmap.c
	modified:   tasks/mm0/src/shm.c
	modified:   tasks/mm0/src/task.c
	new file:   tasks/mm0/src/utcb.c
	modified:   tasks/test0/include/linker.lds

tasks/test0/include/linker.lds

@@ -14,7 +14,7 @@
  */
 /* USER_AREA_START, see memlayout.h */
 virtual_base = 0x10000000;
-__stack = 0x20000000;
+__stack = (0x20000000 - 0x1000 - 8); /* First page before the env/args */
 INCLUDE "include/physical_base.lds"
 
 /* physical_base = 0x228000; */