Conf: Improve handling of keywords

For whatever reason, parser allocated a symbol for every parsed keyword
in each scope. That wasted time and memory. The effect is worsened with
recent changes allowing local scopes, so keywords often promote soft
scopes (with no symbols) to real scopes.

Do not allocate a symbol for a keyword. Take care of keywords that could
be promoted to symbols (kw_sym) and do it explicitly.

nest/config.Y

@@ -154,6 +154,8 @@ CF_ENUM_PX(T_ENUM_AF, AF_, AFI_, IPV4, IPV6)
 
 CF_GRAMMAR
 
+kw_sym: MIN MAX ;
+
 /* Setting of router ID */
 
 conf: rtrid ;