Enable proxies (Self/Parent) for specifying ports.

Significant revamp of Port code.
Some cleanup of SimObject code too, particularly to
make the SimObject and MetaSimObject implementations of
__setattr__ more consistent.
Unproxy code split out of print_ini().

src/python/m5/multidict.py:
    Make get() return None by default, to match semantics
    of built-in dictionary objects.

--HG--
extra : convert_revision : db73b6cdd004a82a08b2402afd1e16544cb902a4

src/python/m5/objects/Device.py

@@ -18,4 +18,4 @@ class BasicPioDevice(PioDevice):
 class DmaDevice(PioDevice):
     type = 'DmaDevice'
     abstract = True
-    dma = Port("DMA port")
+    dma = Port(Self.pio.peerObj.port, "DMA port")