systemd: split "nm-sd.h" out of "nm-sd-adapt.h"

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".

src/dhcp-manager/nm-dhcp-systemd.c

@@ -38,7 +38,6 @@
 #include "sd-dhcp-client.h"
 #include "sd-dhcp6-client.h"
 
-/* we use a private systemd header, thus need to include nm-sd-adapt.h. */
 #include "nm-sd-adapt.h"
 #include "dhcp-lease-internal.h"