Add filter examples for Cancel-Lock/-Key.

This is a possible implementation of Cancel-Lock
and Cancel-Key. cleanfeed.local will check for
a matching Cancel-Key before accepting a cancel,
filter_nnrpd.pl will add Cancel-Locks to all
postings (and a matching Cancel-Key to cancel
messages).

See also
http://th-h.de/blog/archives/1473-INN-Cancel-Lock-und-Cancel-Key.html

Signed-off-by: Thomas Hochstein <thh@inter.net>