Make pollHack work on read-only mounts on Darwin

[valeriangalliat]

Ports commit fee50bf01cd60f66c82493761756b2c345a86aa3 to poll_darwin.go in order to allow read-only mounts on Darwin as well.

Fixes https://github.com/rfjakob/gocryptfs/issues/595 where we can't use gocryptfs -ro or gocryptfs -reverse on macOS because go-fuse tries to write to the read-only mountpoint during initial mount.

Cheers!

Edit: signed the CLA and re-pushed so automated checks pass

Edit: potentially fixes #373

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[hanwen]

could you send me a review through gerrithub? See https://github.com/hanwen/go-fuse/blob/master/CONTRIBUTING

[krim404]

as this is open for a while now and a dealbreaker on OSX - maybe a little bump

[krim404]

merged in https://github.com/hanwen/go-fuse/commit/58a7e145ff0d052afbbf8bb53120572b99c7faaf - can be closed

[valeriangalliat]

Awesome, thanks for the bump and getting this patch though! ❤️