show map timestamp in map info menu

also: more stuff in the map settings menu

getmaptimestamp fmt

  Returns the "last written" timestamp of the current map in the requested
  format. If fmt starts with "U", the time is given as UTC, otherwise
  as local time. If fmt is empty, or not given or just the "U", the format
  defaults to "YYYYMMDD_hh.mm.ss". Use "%c" to get something nicer.

DOCREF getmaptimestamp

source/src/tools.cpp

@@ -26,7 +26,7 @@ const char *timestring(time_t t, bool local, const char *fmt, char *buf)
     struct tm *timeinfo, b;
     timeinfo = local ? localtime_r(&t, &b) : gmtime_r(&t, &b);
 #endif
-    strftime(buf, sizeof(string) - 1, fmt ? fmt : "%Y%m%d_%H.%M.%S", timeinfo); // sortable time for filenames
+    strftime(buf, sizeof(string) - 1, fmt && *fmt ? fmt : "%Y%m%d_%H.%M.%S", timeinfo); // sortable time for filenames
     return buf;
 }