config.py: Adds proxies() function

Since both 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.
utils.py: adds verify_overlay_src() function

api.py: incorporates the verify_overlay_src() function
api.py: Adds overlay type checking

Checks have been added to determine whether or not to readd a repo
if the overlay type reported from the remotedb is different from the
overlay type reported by the local db.
overlay.py: adds update() function

To update local database and overlay source urls, this function
has been created to allow the overlay.type functions to update
their source urls in their own methods.
git.py: adds update() function

Moves the fix_git_source() function to allow both add() and update()
to make use of it. While the update() function calls git remote
set-url, replacing the old url with the new url provided by
overlay.update().
db.py: adds update() function
api.py: incorporates db.update() in api.sync()
svn.py: Adds update() function

Adds _fix_svn_source() function to correct the source url in
order to run the svn switch --relocate command.