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.

source.py: Adds a stub update() function for overlay types
that don't have their own update() function.

config.py: Adds a list of supported overlay types that have their
own update() methods.
db.py: adds update() function
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().
svn.py: Adds update() function

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

Also adds _fix_brz_source() function to be used by add() and update().
mercurial.py: Adds update() function

Also adds _fix_mercurial_source() function to be used by add() and update().
cvs.py: Adds update() function
api.py: incorporates db.update() in api.sync()