systemd.services.<service>.serviceConfig should support multiple entries per option

[Mic92]

Issue description

There are several options like ExecStart, DeviceAllow, or Environment
which can be specified multiple times in systemd units,
where nix only allow one instances due type restrictions.
This leads to ugly workarounds, when multiple entries would be needed (example: #17933, #18822).

The only idea I had, was to support lists as arguments as well,
as single arguments. However this reminds a little bit of perl, where functions act different depending
on the argument type passed, so this may lead to unpleasant surprises.

Any ideas?

[ericsagnes]

I didn't tried with DeviceAllow and Environment, but for ExecStart it is possible to use a list for multiple lines. (In #17933 script is a deliberate choice for the flexibility it brings)

[Mic92]

ups, you are right.