Refactor header status, reason, and target (API Change):

* header::result is a family of functions to replace header::status

* header interface now uses status enum and also ints

* reason-phrase is no longer stored unless the user explicitly
  requests it.

* When serializing, the standard reason is used for the
  corresponding status code unless the user has changed it.

doc/quickref.xml

@@ -88,6 +88,7 @@
             <member><link linkend="beast.ref.http__connection">connection</link></member>
             <member><link linkend="beast.ref.http__error">error</link></member>
             <member><link linkend="beast.ref.http__status">status</link></member>
+            <member><link linkend="beast.ref.http__status_class">status_class</link></member>
             <member><link linkend="beast.ref.http__verb">verb</link></member>
           </simplelist>
           <bridgehead renderas="sect3">Type Traits</bridgehead>