Doing this once makes subsequent operations like garbage collecting
more efficient since we don't have to call makeMutable() first.

http://hydra.nixos.org/build/3661100

Issue #77.

Fixes #77.

Issue #77.

Fixes #76.

Fixes #24.

This should make live easier for single-user (non-daemon)
installations.  Note that when the daemon is used, the "calling user"
is root so we're not using any untrusted caches.

So if a path is not garbage solely because it's reachable from a root
due to the gc-keep-outputs or gc-keep-derivations settings, ‘nix-store
-q --roots’ now shows that root.

But this time it's *obviously* correct!  No more segfaults due to
infinite recursions for sure, etc.

Also, move directories to /nix/store/trash instead of renaming them to
/nix/store/bla-gc-<pid>.  Then we can just delete /nix/store/trash at
the end.

This prevents zillions of derivations from being kept, and fixes an
infinite recursion in the garbage collector (due to an obscure cycle
that can occur with fixed-output derivations).

Waiting for the hook to shut down cleanly sometimes seems to lead to
hangs.

This reverts commit cc511fd6.

Fixes #69.

Fixes issue #70.

If all contending processes wait a fixed amount of time (100 ms),
there is a good probability that they'll just collide again.

Grmbl.

http://hydra.nixos.org/build/3436627

http://hydra.nixos.org/build/3436511

Footnote: doing "make dist-gzip dist-xz" doesn't work with Automake;
you have to do "make dist-gzip; dist-xz".  That's because the dist-*
targets delete the temporary distdir at the end.