Implement a "chain-to" service property.

Implement a "chain-to" service property.

This specifies that if a service completes, another service should be
started (specified by name and resolved at the time it is needed, not
beforehand).

This has applications for recovery ("single-user mode") services, and
for multi-stage booting where service descriptions are on a separate
filesystem which is mounted by a service in an initial stage.