rpc: Implement random-cookie based authentication

When no `-rpcpassword` is specified, use a special 'cookie' file for
authentication. This file is generated with random content when the
daemon starts, and deleted when it exits. Read access to this file
controls who can access through RPC. By default this file is stored in
the data directory but it be overriden with `-rpccookiefile`.

This is similar to Tor CookieAuthentication: see
https://www.torproject.org/docs/tor-manual.html.en

Alternative to #6258. Like that pull, this allows running bitcoind
without any manual configuration. However, daemons should ideally never write to
their configuration files, so I prefer this solution.

src/rpcprotocol.h

@@ -14,6 +14,7 @@
 #include <boost/iostreams/stream.hpp>
 #include <boost/asio.hpp>
 #include <boost/asio/ssl.hpp>
+#include <boost/filesystem.hpp>
 
 #include "json/json_spirit_reader_template.h"
 #include "json/json_spirit_utils.h"
@@ -165,4 +166,13 @@ json_spirit::Object JSONRPCReplyObj(const json_spirit::Value& result, const json
 std::string JSONRPCReply(const json_spirit::Value& result, const json_spirit::Value& error, const json_spirit::Value& id);
 json_spirit::Object JSONRPCError(int code, const std::string& message);
 
+/** Get name of RPC authentication cookie file */
+boost::filesystem::path GetAuthCookieFile();
+/** Generate a new RPC authentication cookie and write it to disk */
+bool GenerateAuthCookie(std::string *cookie_out);
+/** Read the RPC authentication cookie from disk */
+bool GetAuthCookie(std::string *cookie_out);
+/** Delete RPC authentication cookie from disk */
+void DeleteAuthCookie();
+
 #endif // BITCOIN_RPCPROTOCOL_H