On mips64le, syscall.Getdents() and struct syscall.Dirent do
not fit together, causing our Getdents implementation to
return garbage ( https://github.com/rfjakob/gocryptfs/issues/200
and https://github.com/golang/go/issues/23624 ).
Switch to unix.Getdents which does not have this problem -
the next Go release with the syscall package fixes is too
far away, and will take time to trickle into distros.
Now that we have Fstatat we can use it in Getdents to
get rid of the path name.
Also, add an emulated version of getdents for MacOS. This allows
to drop the !HaveGetdents special cases from fusefrontend.
Modify the getdents test to test both native getdents and the emulated
version.
...and if Getdents is not available at all.
Due to this warning I now know that SSHFS always returns DT_UNKNOWN:
gocryptfs[8129]: Getdents: convertDType: received DT_UNKNOWN, falling back to Lstat
This behavoir is confirmed at http://ahefner.livejournal.com/16875.html:
"With sshfs, I finally found that obscure case. The dtype is always set to DT_UNKNOWN [...]"
The Readdir function provided by os is inherently slow because
it calls Lstat on all files.
Getdents gives us all the information we need, but does not have
a proper wrapper in the stdlib.
Implement the "Getdents()" wrapper function that calls
syscall.Getdents() and parses the returned byte blob to a
fuse.DirEntry slice.
