Previously, if a binary cache is hanging/unreachable/slow,
download-from-binary-cache.pl would also hang without any indication
to the user.  Now, if fetching a URL takes more than 5 seconds, it
will print a message to that effect.

This greatly reduces the number of system calls.

Fixes the error "DBD::SQLite::db do failed: column url is not unique".

Amazon S3 returns HTTP status code 403 if a file doesn't exist and the
user has no permission to list the contents of the bucket.  So treat
it as 404 (meaning it's cached in the NARExistence table).

The "$UID != 0" makes no sense: if the local side has write access to
the Nix store (which is always the case) then it doesn't matter if
we're root - we can import unsigned paths either way.

Fixes #118.

Fixes #120.

Functions in Nix are anonymous, but if they're assigned to a
variable/attribute, we can use the variable/attribute name in error
messages, e.g.

while evaluating `concatMapStrings' at `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
...

Fixes #116.

Reported by Pablo Costa.

Otherwise it will set the parent's stdin to non-blocking mode, causing
the subsequent read of the set of inputs/outputs to fail randomly.
That's insane.

Before selecting a machine, build-remote.pl will try to run the
command "nix-builds-inhibited" on the machine.  If this command exists
and returns a 0 exit code, then the machine won't be used.  It's up to
the user to provide this command, but it would typically be a script
that checks whether there is enough disk space and whether the load is
not too high.

This doesn't work if there is no output named "out".  Hydra didn't use
it anyway.

Don't pass --timeout / --max-silent-time to the remote builder.
Instead, let the local Nix process terminate the build if it exceeds a
timeout.  The remote builder will be killed as a side-effect.  This
gives better error reporting (since the timeout message from the
remote side wasn't properly propagated) and handles non-Nix problems
like SSH hangs.

This allows providing additional binary caches, useful in scripts like
Hydra's build reproduction scripts, in particular because untrusted
caches are ignored.

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.