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

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.

Detect a runtime error if the bytes argument is not as expected.

This is based on https://github.com/freebsd-net/packetdrill/pull/15
but adds support for af_arg, getsockopt() and clearing the
acceptfilter.

Allow the last parameter of setsockopt() and getsockopt() calls
to be ....

This was suggested in https://github.com/freebsd-net/packetdrill/pull/15
and a partial implementation was provided.

Sponsored by:	Netlix, Inc.

On FreeBSD IPv6 sockets have the IPV6_V6ONLY option enabled by
default. To allow the scripts to select the address family
via the --ip_version option transparently, disable the
IPV6_V6ONLY option implicilty on all sockets.
This fixes #132.

Sponsored by:	Netflix, Inc.

See https://github.com/google/packetdrill/commit/47582211690ffc2a7f40c371c19d283b2df26a44#diff-9b9398b17ceeb5025735f4c20d63a3e5L547

used by it.

Sponsored by:	Netflix

Don't check optval in getsockopt() when it call failed.
The value makes no sense.
While there, fix a memomry leak.