Sometimes people on #guix ask about a way to change a console keymap,
for example: <https://gnunet.org/bot/log/guix/2015-04-25#T640696>.

So I think it would be good to add a service for loadkeys, what do
people think?

宋文武: This patch is based on your work
<http://lists.gnu.org/archive/html/guix-devel/2014-11/msg00692.html>, so
I tried to credit you in a commit message, but I'm not sure If I did it
right.  Is it OK?