proxy: Switch to blocking I/O with signal handlers to exit.

It's safe to terminate the proxy at any point in its lifecycle, so
there's no point using signalfd() (and the associated select() +
non-blocking I/O gubbins) in it. We might want to use non-blocking
I/O in the future for other reasons, of course, at which point it
might become sensible to use signalfd() again. For now, this makes
us reliably responsive to TERM,INT and QUIT in a way that we weren't
previously.

src/proxy.h

@@ -55,19 +55,16 @@ struct proxier {
 
     /* We transform the raw reply header into here */
 	struct nbd_reply   rsp_hdr;
-
-	/* File descriptor that signal handlers write to */
-	int signal_fd;
 };
 
 struct proxier* proxy_create(
-	int signal_fd,
 	char* s_downstream_address,
 	char* s_downstream_port,
 	char* s_upstream_address,
 	char* s_upstream_port,
 	char* s_upstream_bind );
 int do_proxy( struct proxier* proxy );
+void proxy_cleanup( struct proxier* proxy );
 void proxy_destroy( struct proxier* proxy );
 
 #endif