gnupdate: Add `--attribute' option.
gnupdate: Add `--attribute' option.

* maintainers/scripts/gnu/gnupdate (open-nixpkgs): Add optional
  ATTRIBUTE parameter; honor it.
  (%options)["-A"]: New option.
  (gnupdate)[nixpkgs->snix]: Add ATTRIBUTE parameter and pass it down to
  `open-nixpkgs'.
  [selected-gnu-packages]: New procedure.
  Adjust callers accordingly.

svn path=/nixpkgs/trunk/; revision=30107

NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs,triton/triton,triton/triton,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs