Moves FUSE_USE_VERSION to boxapi.h
Removes extraneous fuse.h includes
Don't free dirname() return value

See http://man7.org/linux/man-pages/man3/dirname.3.html
Merge branch 'FUSE_USE_VERSION' of https://github.com/grigutis/boxfs2 into osxfuse-test
Detect more types of errors at file open
Fixed a couple of warnings