downloads: add basic CVS support

The support is for pserver mode anonymous CVS.
source-check is based on login since many servers don't support or have
ls/rls disabled.

Usage is pretty straightforward.
PKG_SITE defines the site hostname and remote directory.
The module is defined by the bare package name.
Version is date based.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>