Remove usage of `uname -a`

I discovered that this was leaking internal host names, and could potentially
leak other information depending on how a given distro implements uname. It
also just seems less than prudent to shell out from within a gateway.

I've also done a quick review of the code for any other uses of command
execution (`` or %x) and found none.

@jduff @melari please review. @boucher please let us know if you see any
issue with this.