_helpers.c: Remove ugly 'python' junk from argv[0] so 'ps' is prettier.

_helpers.c: Remove ugly 'python' junk from argv[0] so 'ps' is prettier.

Okay, this is pretty gross.  But the 'ps' output was looking ugly, and
now it doesn't.  We remove the 'python' interpreter string and the expanded
pathname of the command being run, so it now shows as (eg.) "bup-join" instead
of "python /blah/blah/blah/cmd/bup-join".

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>