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/proxy.py

@@ -41,7 +41,8 @@ class BaseProxy(object):
 
     def __setattr__(self, attr, value):
         if not attr.startswith('_'):
-            raise AttributeError, 'cannot set attribute on proxy object'
+            raise AttributeError, \
+                  "cannot set attribute '%s' on proxy object" % attr
         super(BaseProxy, self).__setattr__(attr, value)
 
     # support multiplying proxies by constants