su: Make the su package a provider of only the su binary

Additionally, provide su with the base system and remove su from the
util-linux package as it is now provided by shadow.
teamviewer: fix brokenness after updates / GC

Teamviewer puts symlinks to nix store paths in ~/.teamviewer. When those
paths become garbage collected, teamviewer crashes upon start because of
those broken symlinks. An easy workaround to this behaviour is simply to
delete all symlinks before we start teamviewer. Teamviewer will fixup
the symlinks, just like it did the first time the user ran it.
Merge branch 'cache.su' of git://github.com/wkennington/nixpkgs

su: Make the su package a provider of only the su binary

Fixes #1877
Merge branch 'teamviewer-fix-upgrades' of git://github.com/bjornfor/nixpkgs

teamviewer: fix brokenness after updates / GC
stdenv: Use the substitute binary instead of the shell scripted implementation
substitute: Add new package for eventual stdenv inclusion