Add provider-side reload propagation (propagate:reload)

[troglobit]

When a service is reloaded (SIGHUP or reload:script), allow it to declare
that its dependents should also be reloaded/restarted. This is the
provider-side complement to the consumer-side ~ condition modifier
added in #416.

Use case

A target or service that acts as a group manager for several downstream
services. Rather than requiring every dependent to opt in with ~, the
provider declares once that its reload propagates:

service propagate:reload name:netd ...

All services with <pid/netd> or <!pid/netd> conditions would then be
reloaded/restarted when netd reloads, without needing the ~ modifier.

Relationship to other issues

• Add option to reload services in rdeps #416: consumer-side ~ modifier (per-dependent opt-in)
• This issue: provider-side propagate:reload (blanket propagation)

Both mechanisms can coexist. ~ gives fine-grained control on the
dependent side, propagate:reload gives a simple blanket declaration
on the provider side.

Reference

systemd has PropagatesReloadTo= and ReloadPropagatedFrom= for
this purpose. See also systemd/systemd#16443 for discussion around
propagation from target units.