Fix mention_map and hiders causing a light source error

Prevent glyph change messages while loading a level file.
Otherwise a monster hiding under an item triggered a vision recalc
before light sources were linked to their sources, leading into
strange errors looking like pointer corruption.

getlev -> hide_monst -> hideunder -> newsym -> show_glyph ->
 pline_xy -> vision_recalc -> do_light_sources -> get_obj_location

Add in_getlev program state variable, analogous to in_mklev

include/hack.h

@@ -766,6 +766,7 @@ struct sinfo {
     int exiting;                /* an exit handler is executing */
     int saving;                 /* creating a save file */
     int restoring;              /* reloading a save file */
+    int in_getlev;              /* in getlev() */
     int in_moveloop;            /* normal gameplay in progress */
     int in_impossible;          /* reporting a warning */
     int in_docrt;               /* in docrt(): redrawing the whole screen */