support fuse3

[doj]

this supports the fuse3 library on Linux. It's working for me for ~2 years, though my use of encfs is fairly regular file editing. The test suite (not the stress tests, I don't have XFS development files installed on my Linux) are almost all working, only unmounting the encfs file system in "paranoia" mode fails. I've left comments in the test case file about this.

I would like to see this merged into the encfs master branch, along with several other useful pull requests that have been created over the last few years.