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".
As the lldp API changed, adjust "nm-lldp-listener.c".
Note that the commit is not yet functional due to missing
sd_event_source_set_enabled() and sd_event_source_set_time().
This test makes sense because "test-systemd" is not
linked against any other systemd library. So this test
verifies that our libsystemd clone is self contained.
Most interestingly is also, whether we can link libsystemd.a without
having undefined references (which might be wrongly satisfied by an
externally loaded libsystem shared library.
