nix-channel improvements

"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).

corepkgs/unpack-channel.sh

@@ -1,30 +1,4 @@
 mkdir $out
-mkdir $out/tmp
-cd $out/tmp
-
-inputs=($inputs)
-for ((n = 0; n < ${#inputs[*]}; n += 2)); do
-    channelName=${inputs[n]}
-    channelTarball=${inputs[n+1]}
-    
-    echo "unpacking channel $channelName"
-    
-    $bzip2 -d < $channelTarball | $tar xf -
-
-    if test -e */channel-name; then
-        channelName="$(cat */channel-name)"
-    fi
-
-    nr=1
-    attrName=$(echo $channelName | $tr -- '- ' '__')
-    dirName=$attrName
-    while test -e ../$dirName; do
-        nr=$((nr+1))
-        dirName=$attrName-$nr
-    done
-
-    mv * ../$dirName # !!! hacky
-done
-
-cd ..
-rmdir tmp
+cd $out
+$bzip2 -d < $src | $tar xf -
+mv * $out/$name