vm: replace phys avl by array

 . make vm be able to use malloc() by overriding brk()
   and minix_mmap() functions
 . phys regions can then be malloc()ed and free()d instead
   of being in an avl tree, which is slightly faster
 . 'offset' field in phys_region can go too (offset is implied
   by position in array) but leads to bigger code changes
kernel debug: priv can be NULL early on
kernel: vm kernel call can't suspend
include: reduce default stack size
libexec exec fix

	. restores executing shell scripts without explicit interpreter