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

src/restore.c

@@ -1005,6 +1005,8 @@ getlev(NHFILE *nhfp, int pid, xint8 lev)
     short tlev;
 #endif
 
+    gp.program_state.in_getlev = TRUE;
+
     if (ghostly)
         clear_id_mapping();
 
@@ -1233,6 +1235,7 @@ getlev(NHFILE *nhfp, int pid, xint8 lev)
 
     if (ghostly)
         clear_id_mapping();
+    gp.program_state.in_getlev = FALSE;
 }
 
 void