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/util.py

@@ -146,12 +146,15 @@ def write_pid(milterconfig):
     return pid
 
 
-def own_socketfile(milterconfig):
+def own_socketfile(milterconfig, sockname=None):
     """If socket is Unix socket, chown to UserID before dropping privileges"""
     import os
     user, group = user_group(milterconfig.get('UserID'))
     offset = None
-    sockname = milterconfig.get('Socket')
+    if sockname is None:
+        sockname = milterconfig.get('Socket')
+    if sockname is None:
+        return
     if sockname[:1] == '/':
         offset = 0
     elif sockname[:6] == "local:":