Rework getrusage(2) infrastructure

- the userland call is now made to PM only, and PM relays the call to
  other servers as appropriate; this is an ABI change that will
  ultimately allow us to add proper support for wait3() and the like;
  for the moment there is backward compatibility;
- the getrusage-specific kernel subcall has been removed, as it
  provided only redundant functionality, and did not provide the means
  to be extended correctly in the future - namely, allowing the kernel
  to return different values depending on whether resource usage of
  the caller (self) or its children was requested;
- VM is now told whether resource usage of the caller (self) or its
  children is requested, and it refrains from filling in wrong values
  for information it does not have;
- VM now uses the correct unit for the ru_maxrss values;
- VFS is cut out of the loop entirely, since it does not provide any
  values at the moment; a comment explains how it should be readded.

Change-Id: I27b0f488437dec3d8e784721c67b03f2f853120f

etc/system.conf

@@ -61,6 +61,7 @@ service pm
 		EXEC_NEWMEM	# 03
 		WILLEXIT	# 05
 		NOTIFY_SIG	# 39
+		GETRUSAGE	# 47
 		;
 	io	NONE;		# No I/O range allowed
 	irq	NONE;		# No IRQ allowed