python: Backport argparse to older platforms.

python: Backport argparse to older platforms.

Argparse has some convenient advantages over optparse including the
ability to handle optional arguments to flags.  It also supports
parsing arguments as well as options.

This patch copies argparse.py from Python 2.7 into a newly created
compat directory.  It made some very minor syntactic updates in the
process.  Platforms which have a Python version too old to include
argparse by default will have this compat version installed as a
workaround.