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/client.h

@@ -58,6 +58,7 @@ struct client {
 
 };
 
+void client_killswitch_hit(int signal, siginfo_t *info, void *ptr);
 
 void* client_serve(void* client_uncast);
 struct client * client_create( struct server * serve, int socket );