kbfscrypto: distinguish between "darwin" and "ios" in AuthToken
kbfscrypto: distinguish between "darwin" and "ios" in AuthToken

`GOOS` shows `darwin` for both macOS and iOS.  If we want to
distinguish stats from these two on the server side, we should do what
the service does and use `libkb.GetPlatformString()` instead, which
override `GOOS` with "ios" if it's been compiled for iOS.

Issue: KBFS-3010

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client