After doing some fine-tuning of webdav export on Wednesday, I noticed a
problem: There seems to be no way in the webdav spec to delete a collection
(directory) only when it's empty like `rmdir(2)` does. 
It would be possible to check the contents of the collection before
deleting it, but that's complex (involving XML parsing) and race-prone.

So, I decided to add a remote method to delete a directory, and make
git-annex keep track of when a directory in an export is empty, and delete
it. While it does complicate the design some to need to do this, that seems
better than complicating the implementation of remotes like webdav. And
some remotes may not have a `rmdir(2)` equivalent or a way to check if a
directory is empty.

Spent most of today implementing that, including some rather hairy
eSQL to maintain a table of exported directories.

Still not quite done with export..

Today's work was sponsored by Trenton Cronholm on
[Patreon](https://patreon.com/joeyh)