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/__init__.py

@@ -354,7 +354,9 @@ def main():
     Milter.set_flags(Milter.CHGHDRS + Milter.ADDHDRS)
     miltername = 'dkimpy-filter'
     socketname = milterconfig.get('Socket')
-    own_socketfile(milterconfig)
+    if socketname is None:
+        socketname = 'local:/var/run/dkimpy-milter/dkimpy-milter.sock'
+    own_socketfile(milterconfig, socketname)
     drop_privileges(milterconfig)
     sys.stdout.flush()
     Milter.runmilter(miltername, socketname, 240)