3448: unit tests, bug fixes

3448: unit tests, bug fixes

UnixVolume.Delete tests the return status of lockfile (oops, left out of
previous commit)

New unit/integration tests:

* TestPutTouch (volume_unix_test.go): issues a volume.Put for a block
  that already exists, and confirms that the mtime of the existing block
  is updated.

* TestPutBlockTouchFails (keepstore_test.go): issues PutBlock for a
  block that already exists but cannot be modified, confirms that the
  mtime on the old block is unchanged and that a new copy has been
  stored.

Added a volume.Mtime() method to return the block mtime, to assist in
testing.