Normalize coding style by removing consecutive empty lines from C
sources and headers.

https://github.com/NetworkManager/NetworkManager/pull/108

Note that:

 - we compile some source files multiple times. Most notably those
   under "shared/".

 - we include a default header "shared/nm-default.h" in every source
   file. This header is supposed to setup a common environment by defining
   and including parts that are commonly used. As we always include the
   same header, the header must behave differently depending
   one whether the compilation is for libnm-core, NetworkManager or
   libnm-glib. E.g. it must include <glib/gi18n.h> or <glib/gi18n-lib.h>
   depending on whether we compile a library or an application.

For that, the source files need the NETWORKMANAGER_COMPILATION #define
to behave accordingly.

Extend the define to be composed of flags. These flags are all named
NM_NETWORKMANAGER_COMPILATION_WITH_*, they indicate which part of the
build are available. E.g. when building libnm-core.la itself, then
WITH_LIBNM_CORE, WITH_LIBNM_CORE_INTERNAL, and WITH_LIBNM_CORE_PRIVATE
are available. When building NetworkManager, WITH_LIBNM_CORE_PRIVATE
is not available but the internal parts are still accessible. When
building nmcli, only WITH_LIBNM_CORE (the public part) is available.
This granularily controls the build.

Reimport systemd because it uses STRLEN() macro.
We need to when building with -Wvla warning enabled.

Related: https://github.com/systemd/systemd/pull/7625

Systemd instroduces a macro _fallthrough_, see
https://github.com/systemd/systemd/pull/7389.
However, it does not yet seem conclusive how to
handle this properly in ever situation.

While shared/nm-utils/siphash24.c makes use of
the new macro, don't do that in our fork. siphash24.h
does not include all systemd headers, hence _fallthrough_
is not defined. We could re-implement it as _nm_fallthrough,
but given the open questions, that doesn't seem the

Coverity is unhappy about comparing the literal LOG_DEBUG value:

1. Defect type: CONSTANT_EXPRESSION_RESULT
1. NetworkManager-1.9.2/src/systemd/src/libsystemd/sd-event/sd-event.c:2652:
result_independent_of_operands: "7 >= (_level & 7)" is always true regardless
of the values of its operands. This occurs as the logical first operand of "?:".

Work around by instead using an inline function.