vm: change NO_MEM to a more impossible value

vm: change NO_MEM to a more impossible value

fixes an assert() firing when starting X. thanks to the report by pikpik.

	. NO_MEM was 0, which is actually an existing piece
	  of physical memory. it can't be allocated because it's reserved
	  for bios data (by the kernel), but it can be mapped in (e.g.
	  by X), causing sanity check disaster.
	. NONCONTIGUOUS is also obsolete as all allocations are single-page
	  now, i.e. NONCONTIGUOUS is really the default and only mode.