allow cookie, header and stream only in cgi mode.

__header.cpp

@@ -7,6 +7,8 @@ namespace jet {
    
    __header::__header(coreutils::ZString &in, coreutils::MString &parentOut, Global &global, Tag *parent, Tag *local) : Tag(in, parentOut, global, parent, local) {
       output = false;
+      if(!global.cgi)
+	throw coreutils::Exception("must enable cgi mode on jet tag to use header tag.");
       if(!keywordDefined("name"))
 	throw coreutils::Exception("header tag must have name defined.");
       if(!keywordDefined("expr") && keywordDefined("value") && hasContainer)