Minor updates and cleanup for v1.0.1 release

[mtfriesen]

1. Make the installer pick up the xdpapi.dll binary from the xdp subdirectory, i.e. the driver package, which is less likely to be miscopied by human error than the version of xdpapi.dll in the bin root.
2. Make the 1.0 release link point to the 1.0 tree, not just the 1.0.0 tag.
3. Add 1.0.1 to the downlevel test list.
4. Remind releasers to update the downlevel tests.