* Moved bash-specific code from /etc/profile to /etc/bashrc.
* Moved general Bourne shell code from /etc/bashrc to /etc/profile.
* Added "include guards" to both files to ensure that they aren't sourced
multiple times (which would result in lots of redundancy in $PATH, etc.).
* Both files include each other to make sure that the correct system
environment is always defined.
* When the current user has installed the 'bash-completion' package in her
$HOME/.nix-profile, programmable completion is automatically enabled in
interactive shells.
* The /etc/skel/.bashrc we installed has been dropped because it is redundant.
svn path=/nixos/trunk/; revision=29451
