Channels can now advertise a binary cache by creating a file
<channel-url>/binary-cache-url. The channel unpacker puts these in
its "binary-caches" subdirectory. Thus, the URLS of the binary caches
for the channels added by root appear in
/nix/var/nix/profiles/per-user/eelco/channels/binary-caches/*. The
binary cache substituter reads these and adds them to the list of
binary caches.
"nix-channel --add" now accepts a second argument: the channel name.
This allows channels to have a nicer name than (say) nixpkgs_unstable.
If no name is given, it defaults to the last component of the URL
(with "-unstable" or "-stable" removed).
Also, channels are now stored in a profile
(/nix/var/nix/profiles/per-user/$USER/channels). One advantage of
this is that it allows rollbacks (e.g. if "nix-channel --update" gives
an undesirable update).
