Add FreeBSD support to OpenZFS

Add the FreeBSD platform code to the OpenZFS repository.  As of this
commit the source can be compiled and tested on FreeBSD 11 and 12.
Subsequent commits are now required to compile on FreeBSD and Linux.
Additionally, they must pass the ZFS Test Suite on FreeBSD which is
being run by the CI.  As of this commit 1230 tests pass on FreeBSD
and there are no unexpected failures.

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #898 
Closes #8987

README.md

@@ -19,8 +19,6 @@ This repository contains the code for running OpenZFS on Linux and FreeBSD.
 Full documentation for installing OpenZFS on your favorite Linux distribution can
 be found at the [ZoL Site](https://zfsonlinux.org/).
 
-FreeBSD support is a work in progress.  See the [PR](https://github.com/openzfs/zfs/pull/8987).
-
 # Contribute & Develop
 
 We have a separate document with [contribution guidelines](./.github/CONTRIBUTING.md).
@@ -34,3 +32,4 @@ For more details see the NOTICE, LICENSE and COPYRIGHT files; `UCRL-CODE-235197`
 
 # Supported Kernels
   * The `META` file contains the officially recognized supported Linux kernel versions.
+  * Supported FreeBSD versions are 12-STABLE and 13-CURRENT.