* Handle multiple derivations correctly.

  "i686-linux").

  particular happens on distributed builds or when using push/pull.

  print out incorrect "build failed" messages).

  derivation disables scanning for dependencies.  Use at your own
  risk.  This is a quick hack to speed up UML image generation (image
  are very big, say 1 GB).

  It would be better if the scanner were faster, and didn't read the
  whole file into memory.

* Include bootstrap.sh in dist.

  store object just built.

  Now SuSE and Red Hat should yield the same type (`i686-linux').  Mac
  OS X should now give `powerpc-darwin' (i.e., the version number is
  gone).

  system types other than the current system.  I.e., `nix-env -i'
  won't install derivations for other system types, and `nix-env -q'
  won't show them.  The flag `--system-filter SYSTEM' can be used to
  override the system type used for filtering (but not for
  building!).  The value `*' can be used not to filter anything.

  `nix-env -q'.
* Queries can now be combined, e.g., `nix-env -q --status --system'.

  derivations with names matching the derivations being installed.
  The option `--preserve-installed / -P' overrides this behaviour.

  normalisation goal.

  upgrading.

  This fixes a bug reported by Martin:

    $ nix-env -i foo-1.0
    $ nix-env -u foo-1.0
    upgrading foo-1.0 to foo-1.1
  

  Whenever Nix attempts to realise a derivation for which a closure is
  already known, but this closure cannot be realised, fall back on
  normalising the derivation.

  The most common scenario in which this is useful is when we have
  registered substitutes in order to perform binary distribution from,
  say, a network repository.  If the repository is down, the
  realisation of the derivation will fail.  When this option is
  specified, Nix will build the derivation instead.  Thus, binary
  installation falls back on a source installation.  This option is
  not the default since it is generally not desirable for a transient
  failure in obtaining the substitutes to lead to a full build from
  source (with the related consumption of resources).

  much as possible.  (This is similar to GNU Make's `-k' flag.)

* Refactoring to implement this: previously we just bombed out when
  a build failed, but now we have to clean up.  In particular this
  means that goals must be freed quickly --- they shouldn't hang
  around until the worker exits.  So the worker now maintains weak
  pointers in order not to prevent garbage collection.

* Documented the `-k' and `-j' flags.
  

  goal and allow the problem to be handled elsewhere (e.g., at
  top-level).

  expression, we should invalidate it and go back to the derivation
  for which it is a successor.

  their output paths (and only that) on standard output.