{argsparser, cli}.py: adds --readd cli option
overlay.py: Adds overlay type checks for update()
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
config.py: moves EPREFIX constant to top of file