Remove nix.manualNixMachines

Suggested by Marc Weber.  Fixes #1059.

Generate /etc/nix.machines only if buildMachines is not empty.  Thus,
if you want to manage /etc/nix.machines in some other way, you can set
nix.distributedBuilds to true but not set nix.buildMachines.
Rename /etc/nix.machines to /etc/nix/machines

Since we have a /etc/nix directory, we should use it and not pollute
/etc.