conf, tap: Add option to quit once the client closes the connection

This is practical to avoid explicit lifecycle management in users,
e.g. libvirtd, and is trivial to implement.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.h

@@ -196,6 +196,7 @@ struct ctx {
 	char sock_path[UNIX_PATH_MAX];
 	char pcap[PATH_MAX];
 	char pid_file[PATH_MAX];
+	int one_off;
 
 	int pasta_netns_fd;