fuse API stability

Backward compatibility in fuse is guaranteed on multiple levels:

 - the raw kernel interface 
 - the library ABI
 - the library API

While new major versions of the library might come out in the future, the old versions will
still be usable.  Just like gtk-1.2 libraries are still around in distributions, many years
after gtk-2.0 has been released.

I agree though, that in case of drivers where a userspace soultion is not available, keeping
support for old hardware in the kernel is a good thing.