genpoi: add -p/--processes option for parity

People could always set this through the config file, but somehow
not the command line. This commit fixes this oddity by adding the
option to the genPOI script's argument parser.

overviewer_core/aux_files/genPOI.py

@@ -447,6 +447,9 @@ def main():
     parser = ArgumentParser(prog=prog_name)
     parser.add_argument("-c", "--config", dest="config", action="store", required=True,
                         help="Specify the config file to use.")
+    parser.add_argument("-p", "--processes", dest="procs", action="store", type=int,
+                        help="The number of local worker processes to spawn. Defaults to the "
+                        "number of CPU cores your computer has.")
     parser.add_argument("-q", "--quiet", dest="quiet", action="count",
                         help="Reduce logging output")
     parser.add_argument("--skip-scan", dest="skipscan", action="store_true",
@@ -465,6 +468,8 @@ def main():
         mw_parser.parse(args.config)
     except config_parser.MissingConfigException:
         parser.error("The configuration file '{}' does not exist.".format(args.config))
+    if args.procs:
+        mw_parser.set_config_item("processes", args.procs)
     try:
         config = mw_parser.get_validated_config()
     except Exception: