services: shepherd: Add 'shepherd-service-upgrade', from 'guix system'.

services: shepherd: Add 'shepherd-service-upgrade', from 'guix system'.

* guix/scripts/system.scm (service-upgrade): Move to...
* gnu/services/shepherd.scm (shepherd-service-upgrade): ... here.
* tests/system.scm ("service-upgrade: nothing to do", "service-upgrade:
one unchanged, one upgraded, one new", "service-upgrade: service
depended on is not unloaded", "service-upgrade: obsolete services that
depend on each other"): Move to...
* tests/services.scm: ... here.  Adjust to 'service-upgrade' rename.