replace textfilter functions with one-fits-all version

* replaces filtertext() with a version with a dozen of single filters
  that can be used in any combination
* defines classes of allowed characters for strings for various uses
  (see protocol.h line 153ff)
* now the allowed charset of, for example, player names is defined
  in one location only
* fixes issue #58

source/src/clientgame.cpp

@@ -161,7 +161,7 @@ void newname(const char *name)
     if(name[0])
     {
         string tmpname;
-        filtertext(tmpname, name, 0, MAXNAMELEN);
+        filtertext(tmpname, name, FTXT__PLAYERNAME, MAXNAMELEN);
         if(identexists("onNameChange"))
         {
             defformatstring(onnamechange)("onNameChange %d \"%s\"", player1->clientnum, tmpname);