conf-parser: distinguish between multiple sections with the same name

Pass on the line on which a section was decleared to the parsers, so they
can distinguish between multiple sections (if they chose to). Currently
no parsers take advantage of this, but a follow-up patch will do that
to distinguish

[Address]
Address=192.168.0.1/24
Label=one

[Address]
Address=192.168.0.2/24
Label=two

from

[Address]
Address=192.168.0.1/24
Label=one
Address=192.168.0.2/24
Label=two
networkd: add support for [Address] sections

This will allow specifying more options per address than the
simple Address= entry in the [Network] section.

Preliminary support for the same functionality for [Route] sections
are added, but not yet hooked up, as more testing is needed.