* A better substitute mechanism.
Instead of generating a store expression for each store path for
which we have a substitute, we can have a single store expression
that builds a generic program that is invoked to build the desired
store path, which is passed as an argument.
This means that operations like `nix-pull' only produce O(1) files
instead of O(N) files in the store when registering N substitutes.
(It consumes O(N) database storage, of course, but that's not a
performance problem).
* Added a test for the substitute mechanism.
* `nix-store --substitute' reads the substitutes from standard input,
instead of from the command line. This prevents us from running
into the kernel's limit on command line length.
chroot() environment.
* A operation `--validpath' to register path validity. Useful for
bootstrapping in a pure Nix environment.
* Safety checks: ensure that files involved in store operations are in
the store.
Nix. This is to prevent Berkeley DB from becoming wedged.
Unfortunately it is not possible to throw C++ exceptions from a
signal handler. In fact, you can't do much of anything except
change variables of type `volatile sig_atomic_t'. So we set an
interrupt flag in the signal handler and check it at various
strategic locations in the code (by calling checkInterrupt()).
Since this is unlikely to cover all cases (e.g., (semi-)infinite
loops), sometimes SIGTERM may now be required to kill Nix.
