bug#21209: /gnu/store should be able to be a symlink, but it can't

[Kragen Javier Sitaker <kragen@canonical.org>]

Apparently I can’t make /gnu/store a symlink:

    : user@debian:~/pkgs/guix-0.8.1; time /usr/local/bin/guix pull
    guix pull: error: build failed: `/gnu/store' is not a directory

    real	0m0.431s
    user	0m0.104s
    sys	0m0.036s
    : user@debian:~/pkgs/guix-0.8.1; ls -l /gnu/store
    lrwxrwxrwx 1 root root 9 Aug  7 12:56 /gnu/store -> /gnustore
    : user@debian:~/pkgs/guix-0.8.1; ls -Ldl /gnu/store
    drwxrwxr-t 40 root guix-builder 49152 Aug  7 12:56 /gnu/store

Is there a reason for this?  It’s inconvenient.  (My root SSD only has
a couple gigs left.)

I’m working around that by directly mounting the /gnustore partition
under /gnu/store now.

    : user@debian:~/pkgs/guix-0.8.1; sudo vi /etc/fstab
    : user@debian:~/pkgs/guix-0.8.1; grep store /etc/fstab
    /tb/gnustore.ext4 /gnu/store     ext4    loop            2       3
    : user@debian:~/pkgs/guix-0.8.1; sudo umount /gnustore
    : user@debian:~/pkgs/guix-0.8.1; sudo rm /gnu/store
    : user@debian:~/pkgs/guix-0.8.1; sudo mkdir /gnu/store
    : user@debian:~/pkgs/guix-0.8.1; sudo mount -a