Avoids ~180,000 string temporaries created when evaluating a headless
NixOS system.

https://hydra.nixos.org/eval/1434547#tabs-now-fail

This is already handled by the installer.

Also add some examples to nix --help.

Happily the failing tests should prevent anyone from using such a Nix
in situations where they expect sandboxing to be on,
which would otherwise be a risk.

Don't silently succeed seccomp setup when !HAVE_SECCOMP.

Running Nix with build users without seccomp on Linux is dangerous,
and administrators should very explicitly opt-in to it.

configure.ac: check if lzma has MT support, fix deb build/etc.

This is needed for new arches where libseccomp support doesn't exist
yet.

Fixes #1878.

drvName is already assigned to the same value right at the start of the
function.

Boehm guarantees that memory returned by GC_malloc() is zeroed, so take
advantage of that.

Instead of having lexicographicOrder() create a temporary sorted array
of Attr*:s and copying attr names from that, copy the attr names
first and then sort that.

Avoids some malloc() traffic.

Set MANPATH in case man path isn’t set correctly.

Previously, this would fail at startup for non-NixOS installs:

nix-env --help

The fix for this is to just use "nixManDir" as the value for MANPATH
when spawning "man".

To test this, I’m using the following:

$ nix-build release.nix -A build
$ MANPATH= ./result/bin/nix-env --help

Fixes #1627

monitor-fds: Fix on macOS.

Fixes #1871.

Add splitVersion primop.

Fixes #1868.

Allow includes from nix.conf

Enable specifying directories in plugin-files.

installer: create 'enough' build users

Nix stats: flatten statistics

This was caused by derivations with 'allowSubstitutes = false'. Such
derivations will be built locally. However, if there is another
SubstitionGoal that has the output of the first derivation in its
closure, then the path will be simultaneously built and substituted.

There was a check to catch this situation (via pathIsLockedByMe()),
but it no longer worked reliably because substitutions are now done in
another thread. (Thus the comment 'It can't happen between here and
the lockPaths() call below because we're not allowing multi-threading'
was no longer valid.)

The fix is to handle the path already being locked in both
SubstitutionGoal and DerivationGoal.

Flattens the list of statistics as suggested in
https://github.com/NixOS/ofborg/issues/67. This makes it easier to work
with.