The GNU docs describe the `devmajor` and `devminor` fields of the tar
header struct only in the context of character and block special files,
suggesting that in other cases they are not populated. Typical utilities
behave accordingly; this patch teaches `tarfile` to do the same.

Test Plan:
No tests added because none appear to exist for this module. Manually
verified that this enables output that is bit-for-bit compatible with
GNU tar. In particular, this program now passes on my Ubuntu 16.04,
whereas it failed before this patch:

```python
import os
import subprocess
import tarfile
import tempfile

filename = "important_data"
contents = b"The quick brown fox jumps over the lazy dog"

with tempfile.TemporaryDirectory() as tmpdir:
    os.chdir(tmpdir)
    with open(filename, "wb") as outfile:
        outfile.write(contents)
    with tarfile.open("py.tar", "x", format=tarfile.GNU_FORMAT) as outfile:
        outfile.add(filename)
    subprocess.check_call(["tar", "cf", "gnu.tar", filename])
    subprocess.check_call(["shasum", "-a", "256", "py.tar", "gnu.tar"])
    subprocess.check_call(["cmp", "-b", "py.tar", "gnu.tar"])
```

(The exact hashes depend on the calling user and current time but should
always be the same across both output archives.)

wchargin-branch: tarfile-limit-device-headers