MINOR: h2: handle two extra stream states for errors

We need to deal with stream error notifications (RST_STREAM) as well as
internal reporting. The problem is that we don't know in which order
this will be done so we can't unilaterally decide to deallocate the
stream. In order to help, we add two extra stream states, H2_SS_ERROR
and H2_SS_RESET. The former mentions that the stream has an error pending
and the latter indicates that the error was already sent and that the
stream is now closed. It's equivalent to H2_SS_CLOSED except that in this
state we'll avoid sending new RST_STREAM as per RFC7540#5.4.2.

With this it will be possible to only detach or deallocate the h2s once
the stream is closed.

src/mux_h2.c

@@ -87,6 +87,8 @@ enum h2_ss {
 	H2_SS_OPEN,     // open
 	H2_SS_HREM,     // half-closed(remote)
 	H2_SS_HLOC,     // half-closed(local)
+	H2_SS_ERROR,    // an error needs to be sent using RST_STREAM
+	H2_SS_RESET,    // closed after sending RST_STREAM
 	H2_SS_CLOSED,   // closed
 	H2_SS_ENTRIES   // must be last
 } __attribute__((packed));