vsftpd: install sample config file to /etc

vsftpd comes with a sample config file that can serve as a starting
point for customization and enables a basic functionality.

[Peter: drop /usr/share/empty creation]
Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>