Store build logs in /nix/var/log/nix/drvs/<XX>

...where <XX> is the first two characters of the derivation.
Otherwise /nix/var/log/nix/drvs may become so large that we run into
all sorts of weird filesystem limits/inefficiences.  For instance,
ext3/ext4 filesystems will barf with "ext4_dx_add_entry:1551:
Directory index full!" once you hit a few million files.
corepkgs/fetchurl: Enable making the downloaded file executable
build-remote.pl: Don't keep build logs on the build slave
Fix the VM tests
Correctly handle missing logs
Only warn about SQLite being busy once

No need to get annoying.
Improve -I description

Issue #88.
Support the colonies
Nix::Store::derivationFromPath: Return derivation outputs
experimental/hash

adding primop function calculating hash of a string

Signed-off-by: Marc Weber <marco-oweber@gmx.de>
Rename "hash" to "hashString" and handle SHA-1
Make "${./path} ..." evaluate to a string, not a path

Wacky string coercion semantics caused expressions like

  exec = "${./my-script} params...";

to evaluate to a path (‘/path/my-script params’), because
anti-quotations are desuged to string concatenation:

  exec = ./my-script + " params...";

By constrast, adding a space at the start would yield a string as
expected:

  exec = " ${./my-script} params...";

Now the first example also evaluates to a string.
Document ‘hashString’
Enable chroot support on old glibc versions.
Add `Settings::nixDaemonSocketFile'.
build-remote: Use the --quiet flag

‘--option verbosity 0’ doesn't actually do anything.
Security: Don't allow builders to change permissions on files they don't own

It turns out that in multi-user Nix, a builder may be able to do

  ln /etc/shadow $out/foo

Afterwards, canonicalisePathMetaData() will be applied to $out/foo,
causing /etc/shadow's mode to be set to 444 (readable by everybody but
writable by nobody).  That's obviously Very Bad.

Fortunately, this fails in NixOS's default configuration because
/nix/store is a bind mount, so "ln" will fail with "Invalid
cross-device link".  It also fails if hard-link restrictions are
enabled, so a workaround is:

  echo 1 > /proc/sys/fs/protected_hardlinks

The solution is to check that all files in $out are owned by the build
user.  This means that innocuous operations like "ln
${pkgs.foo}/some-file $out/" are now rejected, but that already failed
in chroot builds anyway.
Update release notes
Bump version number
Remove outdated file