Support for New Relic system monitor daemon

These patches add support for the New Relic `nrsysmond` binary as a package named `newrelic_sysmond`, along with a NixOS module. The package only installs the binary while all the configuration is left in the module (I imagine most people won't care about this unless they're running NixOS anyway.)

The daemon runs as user `newrelic` with uid `102`. Using it is as simple as setting `services.nrsysmond.enable` to `true` and `services.nrsysmond.license` to their New Relic license key. Everything else sets a sensible default (including `ssl=true` which, in a bizarre twist, is not enabled by default in their example configuration...)