fix compilation and execution on iOS

This is one of the options to fix Arti to compile for iOS, by no longer importing rust-users when compiling for this platform.

Also fixes issues about running on iOS :

• permissions on the data directory are a bit too lax for fs-misstrust (more specifically, one directory in the path has 775, I haven't verified what group is owning that directory. I could check if requested)
• Instants are ns since startup on iOS (and possibly other platforms, including Linux). Some of the subtractions between Instant and Duration underflow and panic if Arti is run shortly after startup. I've fixed those I triggered, but I think each operation with an Instant ought to be verified.

fix #519 (closed)