Pass and use original sys.argv to/with workers

This gets used e.g. by argparse for the "prog" part.

We could explicitly pass it through and/or set it on
config._parser.prog, but that is a bit tedious just for this use case,
and it looks like "simulating" the main prog here appears to not be that
bad of a hack after all.

xdist/workermanage.py

@@ -2,6 +2,7 @@ from __future__ import print_function
 import fnmatch
 import os
 import re
+import sys
 import threading
 
 import py
@@ -213,6 +214,7 @@ class WorkerController(object):
             "workercount": len(nodemanager.specs),
             "slaveid": gateway.id,
             "slavecount": len(nodemanager.specs),
+            "mainargv": sys.argv,
         }
         # TODO: deprecated name, backward compatibility only. Remove it in future
         self.slaveinput = self.workerinput