pull: Turn ~/.config/guix/current into a symlink to /var/guix/profiles.

This is more consistent with what 'guix package' does, more pleasant for
users (we no longer clobber ~/.config/guix), and more
cluster-friendly (since /var/guix/profiles is usually an NFS share
already.)

* guix/scripts/pull.scm (%current-profile, %user-profile-directory): New
variables.
(migrate-generations, ensure-default-profile): New procedures.
(guix-pull): Use %CURRENT-PROFILE by default.  Call
'ensure-default-profile'.
* doc/guix.texi (Invoking guix pull): Adjust 'guix package -p
~/.config/guix/current' example.
* guix/scripts.scm (warn-about-old-distro): Check %PROFILE-DIRECTORY
"/current-guix".