* 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