slirp: fix ICMP handling on macOS hosts

slirp: fix ICMP handling on macOS hosts

On Linux, SOCK_DGRAM+IPPROTO_ICMP sockets give only the ICMP packet when
read from.  On macOS, however, the socket acts like a SOCK_RAW socket
and includes the IP header as well.

This change strips the extra IP header from the received packet on macOS
before sending it to the guest.  SOCK_DGRAM ICMP sockets aren't
supported on other BSDs, but we enable this behavior for them as well to
treat the sockets the same as raw sockets.

Signed-off-by: Andrew Oates <[email protected]>
Signed-off-by: Samuel Thibault <[email protected]>