Provide Generic RestartPolicy Enum

[jacderida]

• 51ed396 feat!: provide generic RestartPolicy enum

  I found myself in a situation where I need to use the 'always' and 'on success' restart policies on
  Linux Systemd and macOS Launchd. It was not possible to do this while using the generic
  ServiceManager trait; I would have to use specific service managers, which would necessitate the
  introduction of conditional compilation.

  Now we introduce a generic RestartPolicy enum to work with the generic ServiceManager trait.
  This should cover the most important cases supported by most service managers: 'never', 'always',
  and 'on failure', plus the option for 'on success'. Users can still instantiate a particular service
  manager if they want to use specific restart options that are only supported by that manager.

  In some cases these variants don't map directly to some service manager options, so an approximation
  is used and a warning is emitted; however, it should still be very acceptable to configure the
  service in this way since the behaviour should not be too surprising.

  See the changelog entry for more details.

  BREAKING CHANGE: the disable_restart_on_failure field was removed in favour of the restart policy
  and the fields on specific service managers were made optional.

• d0d2bc6 chore(release): bump to version 0.9.0

[jacderida]

Just wanted to note, both of these tests were failing on the last merged PR.

I think the failures are probably unrelated to my changes.

I manually tested creating a user-mode service on macOS as part of my application that uses the crate. It worked fine.

[chipsenkbeil]

@jacderida we can merge and figure out these failures separately. Do you have time to help me look into the macos failure?

[jacderida]

@jacderida we can merge and figure out these failures separately. Do you have time to help me look into the macos failure?

Hey Chip, that's great, thanks. I will merge and publish soon.

I don't have time to look into it right now, but I have holiday from work in about 2 weeks. I promise I will take some time to look into it.