slirp: Propagate host TCP RST to the guest.

slirp: Propagate host TCP RST to the guest.

When the host aborts (RST) its side of a TCP connection we need to
propagate that RST to the guest. The current code can leave such guest
connections dangling forever. Spotted by Jason Wessel.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
[steven@steven676.net: coding style adjustments]
Signed-off-by: Steven Luo <steven+qemu@steven676.net>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>