Warnings for Legacy / Experimental / Obsolete Options

[SJrX]

Some values in the gperf file are marked as legacy or disabled, we should have an inspection that warns about it.
https://github.com/systemd/systemd/blob/master/src/shared/conf-parser.c#L798

[SJrX]

Relates to #67

[SJrX]

https://github.com/systemd/systemd/blob/master/src/core/load-fragment.c#L155

[stephenreay]

It'd be nice if this behaviour could be similar to the native language support behaviour.

i.e. you specify a project-level setting for the systemd version you're targeting, and the plugin will identify properties that are not supported yet, obsolete, etc. I've had more issues with online documentation showing a property (and no notes about introduction version) but then it turns out that version is too new for the distro/release $PROJECT is using.