networkd: fix drop-in conf directory configs overwriting each other

Now we track the sections for example [Address] via line number.
Which was fine till we din't had dropins dir. If we have multiple
sections which have the ideantical line number in diffrent files
we are overwriting these since line number is the key.

This patch fixes this by taking filename and line number as key.
This fixes [Address] and [Route] section overwriting.

fixes: #5141