We have a fork of a lot of useful systemd helper code.
However, until now we shyed away from using it aside from
the bits that we really need.

That means, although we have some really nice implementations
in our source-tree, we didn't use them. Either we were missing
them, or we had to re-implement them.

Add "nm-sd-utils.h" header to very carefully make internal
systemd API accessible to the rest of core.

This is not intended as a vehicle to access all of internal
API. Instead, this must be used with care, and only a hand picked
selection of functions must be exposed. Use with caution, but where it
makes sense.