Partial port to Darwin and FreeBSD

This mainly fixes Linux vs generic Unix differences, with some
differences between Darwin and Freebsd (which are close bit not
identical). Should make fixing for other Unix platforms easier.

Note there are not yet `runc` equivalents for these platforms;
my current use case is image manipulation for the `moby` tool.
However there is interest in OCI runtime ports for both platforms.

Current status is that MacOS can build and run `ctr`, `dist`
and `containerd` and some operations are supported. FreeBSD 11
still needs some more fixes to continuity for extended attributes.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

vendor.conf

@@ -20,7 +20,7 @@ github.com/containerd/btrfs e9c546f46bccffefe71a6bc137e4c21b5503cc18
 github.com/stretchr/testify v1.1.4
 github.com/davecgh/go-spew v1.1.0
 github.com/pmezard/go-difflib v1.0.0
-github.com/tonistiigi/fifo fe870ccf293940774c2b44e23f6c71fff8f7547d
+github.com/tonistiigi/fifo f071cd4a2739654fec4e768a14efd9332b3e8af1
 github.com/urfave/cli 8ba6f23b6e36d03666a14bd9421f5e3efcb59aca
 golang.org/x/net 8b4af36cd21a1f85a7484b49feb7c79363106d8e
 google.golang.org/grpc v1.0.5