services: shepherd: Allow custom 'shepherd' package.

* gnu/services/shepherd.scm (<shepherd-configuration>): New record.
(shepherd-boot-gexp, shepherd-root-service-type): Use it.
(scm->go, shepherd-configuration-file): Allow passing custom
shepherd package.
* gnu/system.scm (operating-system-shepherd-service-names): Use the new
record.
* guix/scripts/system.scm (export-shepherd-graph): Adjust accordingly.
* doc/guix.texi (Shepherd Services). Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>