Add a new [device*] section to NetworkManager.conf. This works similar
like the default connection settings in [connection*].

This will allow us to express per-device configuration in NetworkManager.conf
in our familar style.

Later, via NMConfig's write API it will be possible to make settings
accessible via D-Bus and persist them in NetworkManager-intern.conf.
This way, the user can both edit configuration snippets and modify
them via D-Bus, and also support installing default configuration
from the package.

In a way, a [device*] setting is similar to networkd's link files.
The match options is all encoded in the match-device specs.
One difference is, that the resulting setting can be merged together
by multiple section by partially overwriting them. This makes it
more flexible and allows for example to drop a configuration snippet
that only sets one property, while the rest can be merged from different
snippets.