This reverts commit f78126bf. There
really is no need for such a massive change...

This variable has no reason to exist, given $NIX_STATE_DIR.

If --no-build-output is given (which will become the default for the
"nix" command at least), show the last 10 lines of the build output if
the build fails.

This was added to support Hydra, but Hydra no longer uses it.

This also gets rid of --log-type, since the nested log type isn't
useful in a multi-threaded situation, and nobody cares about the
"pretty" log type.

The flag remembering whether an Interrupted exception was thrown is
now thread-local. Thus, all threads will (eventually) throw
Interrupted. Previously, one thread would throw Interrupted, and then
the other threads wouldn't see that they were supposed to quit.

Otherwise writing to std::cerr is not thread-safe (in particular,
lines will be randomly duplicated).

OpenSSL can randomly segfault unless we register a callback function
to do locking.

https://www.openssl.org/docs/manmaster/crypto/threads.html

Calling a class an API is a bit redundant...

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

Such as whether Nix is built with signed binary cache support, and the
location of the configuration file.

This is mostly useful for hydra-queue-runner.

Borrowed from systemd.

This reverts commit d34d2b2b.

http://hydra.nixos.org/build/17690555

In low memory environments, "nix-env -qa" failed because the fork to
run the pager hit the kernel's overcommit limits. Using posix_spawn
gets around this. (Actually, you have to use posix_spawn with the
undocumented POSIX_SPAWN_USEVFORK flag, otherwise it just uses
fork/exec...)

http://hydra.nixos.org/build/14344391