Listening twice to the same port must not silently discard previous listener

[rixed]

This Hashtbl.replace should really throw an error instead:

mirage-tcpip/lib/tcpip_stack_direct.ml

Line 86 in 9642cc2

else Hashtbl.replace t.tcpv4_listeners port callback

For reference, under Unix listening to an already connected socket raises EINVAL (relevant since there is no separate accept operation in Mirage)