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/sha512/sha512_darwin.go

@@ -17,6 +17,7 @@
 package sha512
 
 import (
+	"hash"
 	"io"
 
 	"crypto/sha512"
@@ -61,3 +62,8 @@ func SumStream(reader io.Reader) ([sha512.Size]byte, error) {
 	copy(returnValue[:], sumSlice)
 	return returnValue, err
 }
+
+// New returns a new hash.Hash computing SHA512.
+func New() hash.Hash {
+	return sha512.New()
+}