passt, tap: Add --fd option

This passes a fully connected stream socket to passt.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
[sbrivio: reuse fd_tap instead of adding a new descriptor,
 imply --one-off on --fd, add to optstring and usage()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.c

@@ -255,7 +255,6 @@ int main(int argc, char **argv)
 
 	quit_fd = pasta_netns_quit_init(&c);
 
-	c.fd_tap = c.fd_tap_listen = -1;
 	tap_sock_init(&c);
 
 	clock_gettime(CLOCK_MONOTONIC, &now);