allow cookie, header and stream only in cgi mode.

tests/test1.jet

@@ -1,4 +1,5 @@
 #!../jet-2.0
 <jet>
 This is output
+<stream name="test" />
 </jet>

tests/testtag.jet

@@ -1,11 +1,19 @@
 #!../jet-2.0
 <jet filterblanklines="true" trimlines="true">
 <tag name="testform">
+ <tag name="field">
+  <input name="field">
+  <container />
+  </input> 
+ </tag>
  <form>
  <container />
  </form>
 </tag>
 <testform>
+ zzz
+ <field>
  xxx
+ </field>
 </testform>
 </jet>