I don't think it's a good idea to use allowedRequisites for stdenv, so
don't mention it here.

These are too difficult to implement via nix-store --serve.

‘--show-progress’ could be re-implemented fairly easily via a
sink/source wrapper class.

‘trusted-users’ is a list of users and groups that have elevated
rights, such as the ability to specify binary caches. It defaults to
‘root’. A typical value would be ‘@wheel’ to specify all users in the
wheel group.

‘allowed-users’ is a list of users and groups that are allowed to
connect to the daemon. It defaults to ‘*’. A typical value would be
‘@users’ to specify the ‘users’ group.

Fixes #294.

If a build log is not available locally, then ‘nix-store -l’ will now
try to download it from the servers listed in the ‘log-servers’ option
in nix.conf. For instance, if you have:

  log-servers = http://hydra.nixos.org/log

then it will try to get logs from http://hydra.nixos.org/log/<base
name of the store path>. So you can do things like:

  $ nix-store -l $(which xterm)

and get a log even if xterm wasn't built locally.

The option '--delete-generations Nd' deletes all generations older than N
days. However, most likely the user does not want to delete the
generation that was active N days ago.

For example, say that you have these 3 generations:

1: <30 days ago>
2: <15 days ago>
3: <1 hour ago>

If you do --delete-generations 7d (say, as part of a cron job), most
likely you still want to keep generation 2, i.e. the generation that was
active 7 days ago (and for most of the past 7 days, in fact).

This patch fixes this issue. Note that this also affects
'nix-collect-garbage --delete-older-than Nd'.

Thanks to @roconnor for noticing the issue!

Grmbl...

It will delete all generations older than the specified number of days.