Support path determination for tradspool rebuilds in tdx-util -R

[eagle]

It's often useful to rebuild a whole set of groups stored in tradindexed, but tdx-util -R requires specifying both the newsgroup and the path, making it impossible to just use xargs or feeding it a list of groups. The path to the newsgroup should be possible to determine based on the normal algorithm and the article spool root, so the path argument to -R should be optional.