Make early protocol auth mechanism generic, and add SRP

Adds everything needed for SRP (and everything works too),
but still deactivated, as protocol v25 init packets aren't final yet.
Can be activated by changing the LATEST_PROTOCOL_VERSION header to 25
inside networkprotocol.h.

src/script/lua_api/l_util.cpp

@@ -30,6 +30,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "tool.h"
 #include "filesys.h"
 #include "settings.h"
+#include "util/auth.h"
 
 // debug(...)
 // Writes a line to dstream