xar: guard against file entries containing multiple name elements (#2236)

xar: guard against file entries containing multiple name elements (#2236)

It appears that there are xar archives (in the form of Apple .pkg files)
that contain TOCs with duplicated name elements:

```xml
<file id="25">
  <data> ... </data>
  <type>file</type>
  <name>PackageInfo</name>
  <name>PackageInfo</name>
  <name>PackageInfo</name>
</file>
```

When libarchive encounters one such file, it will produce an
archive_entry named PackageInfoPackageInfoPackageInfo.

To produce a test archive, the XAR writer was modified to emit two name
elements.