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.