SO_SPLICE implementation

Hello,

Drew Gallatin and I have been working on an implementation of SO_SPLICE,
an interface which allows TCP connections to be spliced together.  This
is intended for use in proxy applications to reduce the overhead of
copying data between connections.

At the moment the interface isn't widely used, though I'll make sure
that net/relayd can take advantage of it (which it already does on
OpenBSD).  If there are existing applications that might take advantage
of it, especially in the FreeBSD base system, I'd be interested to hear
suggestions.

The code review is here if anyone's interested in looking at the
implementation: https://reviews.freebsd.org/D46411