Add in_lua flag

In preparation of making config errors available to lua scripts,
instead outputting them via pline.

include/flag.h

@@ -174,6 +174,7 @@ struct instance_flags {
      * a structure of their own elsewhere some day.
      */
     boolean debug_fuzzer;  /* fuzz testing */
+    boolean in_lua;        /* executing a lua script */
     boolean defer_plname;  /* X11 hack: askname() might not set g.plname */
     boolean herecmd_menu;  /* use menu when mouseclick on yourself */
     boolean invis_goldsym; /* gold symbol is ' '? */