[Merge] lp:~jamesodhunt/upstart/check-for-overlayfs into lp:upstart

[Steve Langasek]

> * Returns: TRUE if any mount point is using overlayfs, else FALSE.

Why warn about *any* overlayfs?  Shouldn't we only care about whether the path to our temp directory is affected by overlayfs?
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https://code.launchpad.net/~jamesodhunt/upstart/check-for-overlayfs/+merge/191393
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