Doc: Add tag for links to RFCs

doc/sbase/dist/birddoc/html/mapping

@@ -205,6 +205,10 @@
 <htmlurl>		"<A HREF=\"[URL]\">[NAME]</A>"
 </htmlurl>
 
+<rfc>			"<A HREF=\"http://www.rfc-editor.org/info/rfc[ID]\">RFC [ID]</A>"
+</rfc>
+
+
 % ref modified to have an optional name field
 <ref>		+	"<@@ref>[ID]\n"
 			"[NAME]</A>\n"

doc/sbase/dist/birddoc/latex2e/mapping

@@ -238,6 +238,9 @@
 <htmlurl>		"\\href{[URL]}{[NAME]}"
 </htmlurl>
 
+<rfc>			"\\href{http://www.rfc-editor.org/info/rfc[ID]}{RFC [ID]}"
+</rfc>
+
 <x>
 </x>
 

doc/sbase/dtd/birddoc.dtd

@@ -99,7 +99,7 @@ anywhere else. <pavel@ucw.cz>
 
 <!-- url added by HG; htmlurl added by esr -->
 <!entity % xref
-	" label|ref|pageref|cite|url|htmlurl|ncite " >
+	" label|ref|pageref|cite|url|htmlurl|rfc|ncite " >
 
 <!entity % inline
 	" (#pcdata | f| x| %emph; |sq| %xref | %index | file )* " >
@@ -501,6 +501,10 @@ anywhere else. <pavel@ucw.cz>
         url cdata #required
         name cdata "&urlnam" >
 
+<!element rfc - o empty>
+<!attlist rfc
+        id cdata #required>
+
 <!element pageref - o empty>
 <!attlist pageref
         id cdata #required>