[NNTP]Cancel post with sha

[rlebeau]

You would simply post a new NNTP message to the server, containing a "Cancel-Key" header that matches a previous message's "Cancel-Lock" header.

See https://www.eyrie.org/~eagle/usefor/draf...ock-01.txt for more details.

How you calculate the values of those headers is outside the scope of Indy. Though Indy does have SHA1 and HMAC classes available.