Hello Guix!

I noticed recently my little dovecot mailserver failed to boot, I
tracked it down to our service installing a symlink as:

  /etc/dovecot/modules -> /run/current-system/profile/lib/dovecot

However, I didn't have the dovecot package globally installed, the
service does not install it AFAICT.

We could extend the service to install dovecot into the global profile,
however instead we can just symlink /etc/dovecot/modules to the dovecot
package in the store directly.

Here's the patch to do that! WDYT?

Thanks,
Pierre