Only listen for monitor connections on localhost by default

This is to prevent accidental exposure of the monitor port to the public
internet if a server admin enables the monitor port without it being
properly firewalled. There is now a config option that lets you override
the address to bind to, so that it can still be made available to other
machines over private networks such as Wireguard.

src/settings.hpp

@@ -38,6 +38,7 @@ namespace settings {
     extern int EVENTMODE;
     extern bool MONITORENABLED;
     extern int MONITORPORT;
+    extern std::string MONITORLISTENIP;
     extern int MONITORINTERVAL;
     extern bool DISABLEFIRSTUSEFLAG;
     extern bool IZRACESCORECAPPED;