For #2.

These are currently failing due to bazillion/fuse@58c769a. See #2.

The tests were recently broken on Linux when default_permissions was set
when mounting. On Linux (but not on OS X), the uid sending the Init
request is root, so all of the files appear to be owned by root. This
was fine until we told the kernel to check permissions.

This fixes cachingfs tests on OS X, where the entry_valid field returned to
fuse is ignored and entries are cached potentially forever.

More info:
    https://github.com/jacobsa/fuse/issues/1
    https://github.com/osxfuse/osxfuse/issues/199

This re-fixes MemFSTest.Mkdir_PermissionDenied on Linux after the recent
shakeup when changing the interface of Mount.