macFUSE 4 support

[elliotkendall]

I'm happy to report that webdavfs can be made to work with the latest macFUSE 4 (4.5.0) as of this writing. You just need two changes: switch from the vanilla bazil.org fuse library to this fork: https://github.com/anacrolix/fuse and remove the AddRoot option. You can see the diff here:

master...elliotkendall:webdavfs:master

I imagine you don't want to do make either of these changes globally, but might you be interested in adding some conditionals for Mac users?