When working on the Terraform / Vault integration I came across the fact
that `Sys().MountConfig(...)` didn't seem to return a response struct,
even though it's a `GET` method.
Looks like just a simple oversight to me. This fix does break API BC,
but the method had no use without its return value so I feel like that's
probably a mitigating factor.
up-to-date information. This allows remount to be implemented with the
same source and dest, allowing mount options to be changed on the fly.
If/when Vault gains the ability to HUP its configuration, this should
just work for the global values as well.
Need specific unit tests for this functionality.
