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.1

@@ -297,6 +297,16 @@ Path for UNIX domain socket used by \fBqemu\fR(1) or \fBqrap\fR(1) to connect to
 Default is to probe a free socket, not accepting connections, starting from
 \fI/tmp/passt_1.socket\fR to \fI/tmp/passt_64.socket\fR.
 
+.TP
+.BR \-F ", " \-\-fd " " \fIFD
+Pass a pre-opened, connected socket to \fBpasst\fR. Usually the socket is opened
+in the parent process and \fBpasst\fR inherits it when run as a child. This
+allows the parent process to open sockets using another address family or
+requiring special privileges.
+
+This option implies the behaviour described for \-\-one-off, once this socket
+is closed.
+
 .TP
 .BR \-1 ", " \-\-one-off
 Quit after handling a single client connection, that is, once the client closes