slirp: Adding ICMPv6 error sending

Adding icmp6_send_error to send ICMPv6 Error messages. This function is
simpler than the v4 version.
Adding some calls in various functions to send ICMP errors, when a
received packet is too big, or when its hop limit is 0.

Signed-off-by: Yann Bordenave <meow@meowstars.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
