Handle defaults for Socket differently

We want to be able to select the default for Socket differently in the
future.

This change augments the API for dkimpy_milter.util.own_socketfile()
by adding an optional sockname argument.  This is a
backward-compatible change.  If we aren't committed to API stability
for this function, we could make a more invasive change that would
probably be a more reasonable API going forward, but this is probably
good enough.

dkimpy_milter/config.py

@@ -39,7 +39,7 @@ defaultConfigData = {
     'SyslogFacility': 'mail',
     'UMask': 0o07,
     'Mode': 'sv',
-    'Socket': 'local:/var/run/dkimpy-milter/dkimpy-milter.sock',
+    'Socket': None,
     'PidFile': '/var/run/dkimpy-milter/dkimpy-milter.pid',
     'UserID': 'dkimpy-milter',
     'Canonicalization': 'relaxed/simple',