Update systemd module to include user units

[KarolosLykos]

Issue: #294

Added an optional busType parameter in the Service function on the systemd module in order to not introduce a breaking change.

I tested locally and seems to work just fine although, I would really appreciate any ideas on how to write tests for this.

[soumya92]

Since there are only two bus types, I think it would be cleaner to add UserService and UserTimer methods at top-level. WDYT?

[KarolosLykos]

Since there are only two bus types, I think it would be cleaner to add UserService and UserTimer methods at top-level. WDYT?

I don't know..., adding UserService and UserTimer methods at the top-level it may result in unnecessary duplication. Additionally, there is another bus used in the tests :D, which would require further modifications.

[soumya92]

The method bodies can just forward to a private method, that one you can freely change the api for. I just don't like the idea of using varargs to workaround the lack of overloads/optional arguments in go, especially in the public api.

It can lead to calls like Timer('foo', System, User), which would compile but only use the system bus. We could work around that by panic if len(busTypes) > 1, but might be easier to just avoid that by using two more top-level functions in the API.

[KarolosLykos]

The method bodies can just forward to a private method, that one you can freely change the api for. I just don't like the idea of using varargs to workaround the lack of overloads/optional arguments in go, especially in the public api.

It can lead to calls like "Timer('foo', System, User), which would compile but only use the system bus. We could work around that by panic` if len(busTypes) > 1, but might be easier to just avoid that by using two more top-level functions in the API.

Hi @soumya92 , updated in the last commit. Waiting for your feedback. Cheers