Users outside of src/systemd should not make use of internal API.
Currently, "nm-dhcp-systemd.c" still makes use of internal systemd
functions. Instead of letting "nm-dhcp-systemd.c" include internal
headers, handpick the required defines to "nm-sd.h" and hide "nm-sd-adapt.h".
"nm-sd-adapt.h" is now only used to compile internal systemd sources.
Now we have:
"nm-sd.h" is a header file of NetworkManager with utilities
related to systemd. It can be used anywhere freely.
Also, systemd headers that are considered public API (like
"sd-event.h") can be used without restrictions.
When compiling the systemd sources, we always must include
"nm-sd-adapt.h" as first. Similarly, systemd headers must
not include "nm-sd-adapt.h", because they are either public
(in which case the adapter is not needed) or they are internal
(in which case they are themself included via a systemd source).
Sometimes, we must internal API (like "dhcp-lease-internal.h").
In this case, we also must include "nm-sd-adapt.h".
