groff: set "operating system" for mdoc pages
groff: set "operating system" for mdoc pages

Prior to this change, man pages from Nixpkgs written using the mdoc(7)
macros would start like this:

    NC(1)                   BSD General Commands Manual                   NC(1)

and end like this:

    BSD                          December 27, 2018                          BSD

No matter what operating system they were run on.

It's far more accurate to say "Nixpkgs General Commands Manual", so
with this patch we configure groff to do just that.  The variable is
called "operating-system", but I think it makes more sense to say
"Nixpkgs" than "NixOS" or something, because packages from Nixpkgs can
run on lots of operating systems, and the important thing is that the
package is from Nixpkgs.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs