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>
Even though [PEP 8 doesn't explicitly prohibit blank lines directly after function definitions](http://legacy.python.org/dev/peps/pep-0008/#blank-lines), it's fairly obvious that it's against the spirit of the guidelines, as well as certainly being inconsistent with the vast majority of Python code out there.