http2: use CURL_HTTP_VERSION_* symbols instead of NPN_*

Since they already exist and will make comparing easier

lib/urldata.h

@@ -600,12 +600,6 @@ enum upgrade101 {
   UPGR101_WORKING             /* talking upgraded protocol */
 };
 
-enum negotiatenpn {
-  NPN_INIT,                   /* default state */
-  NPN_HTTP1_1,                /* HTTP/1.1 negotiated */
-  NPN_HTTP2                   /* HTTP2 (draft-xx) negotiated */
-};
-
 /*
  * Request specific data in the easy handle (SessionHandle).  Previously,
  * these members were on the connectdata struct but since a conn struct may
@@ -1067,7 +1061,7 @@ struct connectdata {
   } tunnel_state[2]; /* two separate ones to allow FTP */
   struct connectbundle *bundle; /* The bundle we are member of */
 
-  enum negotiatenpn negnpn;
+  int negnpn; /* APLN or NPN TLS negotiated protocol, CURL_HTTP_VERSION* */
 };
 
 /* The end of connectdata. */