BUG/MEDIUM: stconn/mux-h1: Fix suspect change causing timeouts

This fixes an issue I've had where if a connection was idle for ~23s
it would get in a bad state.  I don't understand this code, so I'm
not sure exactly why it was failing.

I discovered this by bisecting to identify the commit that caused the
regression between 2.9 and 3.0.  The commit is
d2c3f8dde7: "MINOR: stconn/connection:
Move shut modes at the SE descriptor level" - a part of v3.0-dev8.
It seems to be an innocent renaming, so I looked through it and this
stood out as suspect:

    -        if (mode != CO_SHW_NORMAL)
    +        if (mode & SE_SHW_NORMAL)

It looks like the not went missing here, so this patch reverses that
condition.  It fixes my test.

I don't quite understand what this is doing or is for so I can't write
a regression test or decent commit message.  Hopefully someone else
will be able to pick this up from where I've left it.

[CF: This inverts the condition to perform clean shutdowns. This means no
     clean shutdown are performed when it should do. This patch must be
     backported to 3.0]

src/mux_h1.c

@@ -4376,7 +4376,7 @@ static void h1_shut(struct stconn *sc, enum se_shut_mode mode, struct se_abort_i
 
   do_shutw:
 	h1_close(h1c);
-	if (mode & SE_SHW_NORMAL)
+	if (!(mode & SE_SHW_NORMAL))
 		h1c->flags |= H1C_F_SILENT_SHUT;
 
 	if (!b_data(&h1c->obuf))