/proc/pid/checkpointable

Started out implementing '/proc/pid/checkpointable' which reports 0 or 1
depending on whether a process is checkpointable atm. But realized hat
some patches that were discussed earlier are missing from Dave Hansen's
tree.

This patchset includes those missing patches plus two new patches that
create /proc/pid/checkpointable.

	[PATCH 1/6] Checkpoint multiple processes
	[PATCH 2/6] Restart multiple processes
	[PATCH 3/6] Check 'may_checkpoint()' early
	[PATCH 4/6] Deny external checkpoint unless task is frozen
	[PATCH 5/6] Define and use proc_pid_checkpointable()
	[PATCH 6/6] Explain reason for task being uncheckpointable