The $channelName variable passed to the channel builder is the last
portion of the URL and while that works in the previous test for
channels prior to #519, it doesn't work if the last portion is
nixexprs.tar.bz2.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
"nix-channel --add" now accepts a second argument: the channel name.
This allows channels to have a nicer name than (say) nixpkgs_unstable.
If no name is given, it defaults to the last component of the URL
(with "-unstable" or "-stable" removed).
Also, channels are now stored in a profile
(/nix/var/nix/profiles/per-user/$USER/channels). One advantage of
this is that it allows rollbacks (e.g. if "nix-channel --update" gives
an undesirable update).
