Our internal copy of systemd should not be in the search path.
Instead, let users only
#include "systemd/nm-sd.h"
which then includes everything from systemd that we need.
We want to avoid to accidentally include anything from our
systemd-copy. Any user of that should only include "nm-sd.h",
which then includes everything that is needed further.
For example, "src/devices/wwan/nm-modem-manager.c" has
#include <systemd/nm-daemon.h>
which in turn includes
#include "_sd-common.h"
This works all correctly before, because #include "" will first
look in the directory where sd-daemon.h is. However, our mixing of
external systemd library and internal copy is rather dangerous.
Try to avoid it further by keeping the include paths clean.
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".
