Added SETURIPRESENT and SETURIMISSING to external special remote protocol

Useful for things like ipfs that don't use regular urls.

An external special remote can add a regular url to a key, and then
git-annex get will download it from the web. But for ipfs, we want to
instead tell git-annex that the uri uses OtherDownloader. Before this
change, the external special remote protocol lacked a way to do that.

doc/special_remotes/external/git-annex-remote-ipfs

@@ -89,7 +89,7 @@ while read line; do
 					if [ -z "$addr" ]; then
 						echo TRANSFER-FAILURE STORE "$key" "ipfs add failed"
 					else
-						echo "SETURLPRESENT" "$key" "$(addresstourl "$addr")"
+						echo "SETURIPRESENT" "$key" "$(addresstourl "$addr")"
 						echo TRANSFER-SUCCESS STORE "$key"
 					fi
 				;;