fuse/test: don't dereference results if FS calls return error.
fuse/test: rename delete/notify test so they are only exercised on Linux.
fuse: use Unmount syscall on darwin directly.
fuse: on Darwin always return OK for STATFS.

OSXFUSE requires STATFS to always be present.
fuse: on darwin, read the FUSE device with a single goroutine.

This is necessary to work around apparent bugs in the OSX FUSE
implementation.