Do not create PidFile by default

By default, avoid creating a PIDFile.

PIDFiles are racy and potentially dangerous.  Modern system
supervision systems don't need them, because they manage the process
groups directly.

If the configuration file doesn't specify a PidFile, dkimpy-milter
shouldn't try to create one.

man/dkimpy-milter.conf.5

@@ -338,7 +338,7 @@ will be checked. [PeerList NOT IMPLEMENTED - included for reference only]
 .TP
 .I PidFile (string)
 Specifies the path to a file that should be created at process start
-containing the process ID.
+containing the process ID.  If not specified, no such file will be created.
 
 .TP
 .I Selector (string)