dh_installsystemd command line arguments handling

[Niels Thykier]

Daniele Nicolodi:
> [...]
>> In the interpretation, a call to "dh_installsystemd [--option]
>> foo.service" could make sense[1] even if the helper acted on multiple
>> binaries (as a "generate only maintscripts for foo.service subject to
>> --option").
>>   Though at the same time, in that interpretation "foo.service" is
>> definitely sufficient to reference the unit file as we must always
>> reference an "installed" unit file (as you mentioned yourself).
> 
> I fixed the confusion in dh_installsystemd about unit names ans unit
> paths and now it works consistently, see merge request on salsa. I'm
> however not convinced that guessing which binary package to operate on
> based on the unit name is a good idea or worth the extra complication.
> Thus that is not implemented. It is not impossible to do if someone
> insists that it is a good idea.
> 
> Cheers,
> Dan
> 

We could align it with what other helpers do and have arguments only
apply to $dh{FIRSTPACKAGE} by default (with --all making it work on all
packages).

Then "dh_installsystemd foo.service" will be well-defined and work for
any packages while still enabling overriding the default (via "-ppkg").

Thanks,
~Niels