flexnbd serve: Make the killswitch per-client-thread

This is a bit tricky, but calling shutdown() on a socket in a signal
handler is safe, and (at least in linux) appears to cause any read()
or write() calls blocked on that socket to return, even with SA_RESTART.

I'm not confident enough about the rest of flexnbd's syscall error
handling to turn SA_RESTART off for this signal...

src/flexnbd.h

@@ -5,6 +5,7 @@
 #include "mirror.h"
 #include "serve.h"
 #include "proxy.h"
+#include "client.h"
 #include "self_pipe.h"
 #include "mbox.h"
 #include "control.h"