config.py: Adds proxies() @property function

Since remotedb.py and tar.py both made use of the same code
that determined available proxies, the code has been moved to a
function in config.py and both files have been changed to make use
of this function as a value.
api.py: fixes success assignment in delete_repos()
api.py: fixes byte string output for update_news()
config.py: moves EPREFIX constant to top of file