Fix OS X build

- Explicitly cast statfs_t members to int64 (this structure is
   platform-specific)
 - Add pass-through New methods to Darwin SHA package
 - Move scsi pkg types to common translation unit (package was empty)
 - Add stub implementations mount/disk ops for OS X

pkg/crypto/sha256/sha256_darwin.go

@@ -17,6 +17,7 @@
 package sha256
 
 import (
+	"hash"
 	"io"
 
 	"crypto/sha256"
@@ -45,3 +46,8 @@ func Sum(reader io.Reader) ([]byte, error) {
 	}
 	return d.Sum(nil), nil
 }
+
+// New returns a new hash.Hash computing SHA256.
+func New() hash.Hash {
+	return sha256.New()
+}