SHA256 was chosen because it is reasonably standard, the file names
don't explode in length (this rules out SHA512) and it is supported by
basically all versions of OpenSSL (this rules out SHA512/256 and SHA3).
The protocol is changed in a backward compatible way: The supporting
server sends the SHA256 corresponding to the map in the `MAP_DETAILS`
message prior to sending the `MAP_CHANGE` message. The client saves the
SHA256 obtained from the `MAP_DETAILS` message until the next
`MAP_CHANGE` message.
For servers not supporting this protocol, the client falls back to
simply opening maps like in the previous scheme.
Remove the `map_version` tool, it is not being used and would have been
a little bit effort to update.
Use the OpenSSL implementation of SHA256 if it is supported, otherwise
fall back to a public domain one.
Fix#1127.
This uses an extended protocol message to signal that rcon
authentication might require a username, allowing the client to enable
the username prompt.
Add a forgotten logout on key update
This system can easily be extended by independent authors without
collisions, something the old system with plain increasing integers did
not allow.
Do this by utilizing the previously unused message code `NETMSG_NULL`
which has a value of 0.
This works for engine and game messages, snapshot items and events.
