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.

changelog/388.feature.rst

@@ -0,0 +1,3 @@
+``mainargv`` is made available in ``workerinput`` from the host's ``sys.argv``.
+
+This can be used via ``request.config.workerinput["mainargv"]``.