Initial addition of 9p code to Podman

This includes two new hidden commands: a 9p server,
`podman machine server9p`, and a 9p client,
`podman machine client9p` with `server9p` currently only
configured to run on Windows and serve 9p via HyperV vsock, and
`client9p` only configured to run on Linux. The server is run by
`podman machine start` and has the same lifespan as gvproxy
(waits for the gvproxy PID to die before shutting down). The
client is run inside the VM, also by `podman machine start`, and
mounts uses kernel 9p mount code to complete the mount. It's
unfortunately not possible to use mount directly without the
wrapper; we need to set up the vsock and pass it to mount as an
FD.

In theory this can be generalized so that the server can run
anywhere and over almost any transport, but I haven't done this
here as I don't think we have a usecase other than HyperV right
now.

[NO NEW TESTS NEEDED] This requires changes to Podman in the VM,
so we need to wait until a build with this lands in FCOS to test.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

vendor/github.com/mdlayher/vsock/fd_linux.go

@@ -0,0 +1,36 @@
+package vsock
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// contextID retrieves the local context ID for this system.
+func contextID() (uint32, error) {
+	f, err := os.Open(devVsock)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	return unix.IoctlGetUint32(int(f.Fd()), unix.IOCTL_VM_SOCKETS_GET_LOCAL_CID)
+}
+
+// isErrno determines if an error a matches UNIX error number.
+func isErrno(err error, errno int) bool {
+	switch errno {
+	case ebadf:
+		return err == unix.EBADF
+	case enotconn:
+		return err == unix.ENOTCONN
+	default:
+		panicf("vsock: isErrno called with unhandled error number parameter: %d", errno)
+		return false
+	}
+}
+
+func panicf(format string, a ...interface{}) {
+	panic(fmt.Sprintf(format, a...))
+}