services: Use 'file-append' in more places.
* gnu/services/avahi.scm (avahi-shepherd-service): Use #$(file-append x y)
instead of (string-append #$x y).
* gnu/services/base.scm (unicode-start)
(console-keymap-service-type, console-font-shepherd-services)
(mingetty-shepherd-service, nscd-shepherd-service)
(hydra-key-authorization, guix-shepherd-service)
(guix-publish-shepherd-service, udev-shepherd-service)
(gpm-shepherd-service, <kmscon-configuration>)
(kmscon-service-type): Likewise.
* gnu/services/shepherd.scm (shepherd-boot-gexp): Likewise.