Although a peer address MUST be specified for IPv4, it MUST NOT
be specified when using IPv6, when configuring the IP address
on an utun device using ifconfig.

This fixes a bug which did not show up on FreeBSD, but on MacOS.

This fixes a bug which did not show up on FreeBSD, but on MacOS.

This requires the tunneling devices to be used in P2P mode. So
pass the gateway address around.
This is not working:
* Can't configure IPv6 addresses on utun devices.
* Can't use libpcap on utun devices
* Can't use libpcap as currently being used (taking the direction
  into account.

MacOS provides an older version of bison which results in
macro redifinitions for CMSG_LEN and CMSG_DATA. Renaming
them resolves this.

Still missing: An implementation of is_thread_sleeping() for MacOS.

MacOS uses a different name for the TCP_INFO socket option.

Use the portable pcap_set_immediate_mode() function instead of
BIOCIMMEDIATE.

Thanks to Timo Voelker for finding and reporting the bug.

When using TFO and the SYN carries user data, the sending of
the SYN-ACK is delayed and accept returns. This allows the
application to provide user data to be put in the SYN-ACK.

Use the IANA assigned option number for the TCP Fast Open option.
Use EXP-FO instead of TO for using the deprecate experimental option.