services: shepherd: Make 'shepherd-configuration-file' non-monadic.

Suggested by atw on #guix.

* gnu/services/shepherd.scm (shepherd-service-file): Use 'scheme-file'
instead of 'gexp->file'.
(shepherd-configuration-file): Likewise, and adjust to non-monadic
style.
(shepherd-boot-gexp): Adjust accordingly.
* guix/scripts/system.scm (upgrade-shepherd-services): Use
'lower-object' in addition to 'shepherd-service-file'.