Fix all calls to tempfile.mkdtemp to osutils.mkdtemp.

* bzrlib/transform.py:
(TransformPreview.__init__): Use osutils.mkdtemp instead of
tempfile.mkdtemp.

* bzrlib/tests/test_whitebox.py:
(MoreTests.test_relpath): Use osutils.mkdtemp instead of
tempfile.mkdtemp.

* bzrlib/tests/test_setup.py:
(TestSetup.test_build_and_install): Use osutils.mkdtemp instead of
tempfile.mkdtemp.

* bzrlib/tests/test_bundle.py:
(BundleTester.get_checkout): Use osutils.mkdtemp instead of
tempfile.mkdtemp.

* bzrlib/tests/blackbox/test_outside_wt.py:
(TestOutsideWT.test_cwd_log,
TestOutsideWT.test_diff_outside_tree): Use osutils.mkdtemp instead
of tempfile.mkdtemp.

* bzrlib/smart/repository.py:
(SmartServerRepositoryTarball._copy_to_tempdir): Use
osutils.mkdtemp instead of tempfile.mkdtemp.
(SmartServerRepositoryTarball._tarfile_response): Line too long.

* bzrlib/remote.py:
(RemoteRepository._copy_repository_tarball): Use osutils.mkdtemp
instead of tempfile.mkdtemp.

* bzrlib/osutils.py:
(_mac_mkdtemp): Add docstring.

* bzrlib/mail_client.py:
(ExternalMailClient.compose): Use osutils.mkdtemp instead of
tempfile.mkdtemp.

* bzrlib/diff.py:
(DiffFromTool.__init__): Use osutils.mkdtemp instead of
tempfile.mkdtemp.