add a map license and comment string to the map header

Two new string variables, mapinfo_license and mapinfo_comment, are stored
withing the header of a map file.

The variables are automatically read and written during loading and
saving maps. Default values are empty strings. The comment string is
ignored, as long as no license string is set. The license string is meant
to hold an abbreviated name of the license, picked from a list of
known map licenses.

DOCREF mapinfo_license mapinfo_comment

source/src/clientgame.cpp

@@ -1085,6 +1085,7 @@ void resetmap(bool mrproper)
     deleted_ents.setsize(0);
     pinnedclosestent = false;
     particlereset();
+    setmapinfo("", "");
     if(mrproper)
     {
         audiomgr.clearworldsounds();