The `noChmod` option is deprecated, and replaced with the `chmod`
option. The default is now to *not* explicitly set the mode of created
filesystem entries.

This is a deviation from `tar(1)` implementations, but prevents the need
to call `process.umask()`, which is deprecated and not thread safe.

To still set modes explicitly, while avoiding the need to call
`process.umask()`, a `processUmask` numeric option may now be provided
along with `chmod: true`.

This replaces a lot of manual mkdir/rimraf with t.testdir()

Fix: #404

tar-stream creates some interesting tarballs, but they are technically
allowed, and should be handled properly.

Fix: #312

Also, this cleans up a flaky race condition in the unpack test.