p2p --link now defaults to setting up a bi-directional link

Both the local and remote git repositories get remotes added
pointing at one-another.

Makes pairing twice as easy!

Security: The new LINK command in the protocol can be sent repeatedly,
but only by a peer who has authenticated with us. So, it's entirely safe to
add a link back to that peer, or to some other peer it knows about.
Anything we receive over such a link, the peer could send us over the
current connection.

There is some risk of being flooded with LINKs, and adding too many
remotes. To guard against that, there's a hard cap on the number of remotes
that can be set up this way. This will only be a problem if setting up
large p2p networks that have exceptional interconnectedness.

A new, dedicated authtoken is created when sending LINK.

This also allows, in theory, using a p2p network like tor, to learn about
links on other networks, like telehash.

This commit was sponsored by Bruno BEAUFILS on Patreon.

P2P/Address.hs

@@ -14,6 +14,7 @@ import Git.Types
 import Creds
 import Utility.AuthToken
 import Utility.Tor
+import qualified Utility.SimpleProtocol as Proto
 
 import qualified Data.Text as T
 
@@ -46,6 +47,10 @@ instance FormatP2PAddress P2PAddress where
 			return (TorAnnex (OnionAddress onionaddr) onionport)
 		| otherwise = Nothing
 
+instance Proto.Serializable P2PAddressAuth where
+	serialize = formatP2PAddress
+	deserialize = unformatP2PAddress
+
 torAnnexScheme :: String
 torAnnexScheme = "tor-annex:"