Problem noticed by niksnut.

Based on https://github.com/NixOS/nix/pull/6 from shlevy

For instance, this prevents paths from being deleted that are in use
by a "nix-build --run-env" session.

Reported by Peter Simons.

This reverts commit 28bba8c4.

‘--option verbosity 0’ doesn't actually do anything.

http://hydra.nixos.org/build/3661100

This should make live easier for single-user (non-daemon)
installations.  Note that when the daemon is used, the "calling user"
is root so we're not using any untrusted caches.

For example, given a derivation with outputs "out", "man" and "bin":

  $ nix-build -A pkg

produces ./result pointing to the "out" output;

  $ nix-build -A pkg.man

produces ./result-man pointing to the "man" output;

  $ nix-build -A pkg.all

produces ./result, ./result-man and ./result-bin;

  $ nix-build -A pkg.all -A pkg2

produces ./result, ./result-man, ./result-bin and ./result-2.

Binary caches can now specify a priority in their nix-cache-info file.
The binary cache substituter checks caches in order of priority.  This
is to ensure that fast, static caches like nixos.org/binary-cache are
processed before slow, dynamic caches like hydra.nixos.org.

This allows disabling the use of binary caches, e.g.

  $ nix-build ... --option use-binary-caches false

Note that

  $ nix-build ... --option binary-caches ''

does not disable all binary caches, since the caches defined by
channels will still be used.

It's a mystery why this error is not triggered in the build farm
(e.g. http://hydra.nixos.org/build/3265602).  Ah well.

If ‘--link’ is given, nix-push will create hard links to the NAR files
in the store, rather than copying them.  This is faster and requires
less disk space.  However, it doesn't work if the store is on a
different file system.

This prevents unnecessary and slow rebuilds of NARs that already exist
in the binary cache.

I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).

This operation allows fixing corrupted or accidentally deleted store
paths by redownloading them using substituters, if available.

Since the corrupted path cannot be replaced atomically, there is a
very small time window (one system call) during which neither the old
(corrupted) nor the new (repaired) contents are available.  So
repairing should be used with some care on critical packages like
Glibc.

Commit 6a214f3e copied most of the Nix
shell initialisation code from NixOS to nix-profile.sh; however, that
code assumes a multi-user install and is Linux-specific (e.g. it calls
the "stat" command).  So go back to the simple single-user version.

Fixes #49.

Negative lookups are purged from the DB after a day, at most once per
day.  However, for non-"have" lookups (e.g. all except "nix-env
-qas"), negative lookups are ignored after one hour.  This is to
ensure that you don't have to wait a day for an operation like
"nix-env -i" to start using new binaries in the cache.

Should probably make this configurable.