Fix how we are handling system services in RPM packages. (#14781)

* Drop non-systemd init code in RPM spec file.

All of our currently supported RPM-based platforms are systemd-based, so
this is functionally dead code.

* Fix how we handle systemd service installation.

- Rely on the platform-provided macros, instead of doing our own thing.
  These do the right thing in 99.99% of cases, and behave how we
  actually need them to.
- Don’t depend on systemd at runtime. The macros handle this correctly,
  and nothing we actually do _needs_ systemd at runtime.
- Actually follow packaging guidelines for upgrades. In particular, only
  restart the agent if it’s already running (instead of unconditionally),
  and don’t enable it if it’s disabled.
- Just put the macro invocations down in the actual scriptlets, instead
  of using complicated nested macro definitions to compute things ahead
  of time. This makes the code _much_ easier to understand.

* Add a systemd preset file so the service is enabled by default.

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