This exposes Broker's new WebSocket support in Zeek. To enable it,
call `Broker::listen_websocket()`. Zeek will then start listening on
port 9997 for incoming WebSocket connections.
See the Broker documentation for a description of the message format
expected over these WebSocket connections.
SHA1 signatures are only allowed in the legacy policy. I have no idea
where this is documented - I found it in a patch for the OpenSSL source
package.
Also adding tar as a specific dependency, since we have a test that
requires it, and at least for me it was not present in a minimal
installation.
Fixes#2115
