Make specification of CVS module to convert optional.

If we're inside a checked out CVS repository, there is
no need to explicitly specify the module as it is
available in CVS/Repository.
Also read CVS/Root if it's available and -d is not specified.
Finally, explicitly pass root to cvsps as CVS/Root takes
precedence over CVSROOT.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>