slirp: allow host port 0 for hostfwd

The OS will allocate automatically a free port. This is useful if you
want to be sure to not get any port conflict. You still have to figure
out which port you got, for example with "lsof" (this could be exposed
in the monitor if needed).

Example of use:

     $ qemu-system-x86_64 -net user,hostfwd=127.0.0.1:0-:22 ...

Then, get your port with:

     $ lsof -np 1474 | grep LISTEN
     qemu-syst 31777 bernat 12u IPv4 [...] TCP 127.0.0.1:35145 (LISTEN)

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>