Python rewrite of tar_scm

This is a rewrite of the tar_scm service in Python to improve security and
restore functionality after commit acecb041929bc8210c4165574e85bd8a6f0748b2.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
Having said that, it probably doesn't make sense for me to go through this whole file looking for PEP8 violations - we should just run an automated check on it.