NetworkManager is single-threaded and uses a mainloop.

However, sometimes we may need multiple threads. For example, we will
need to write sysctl values asynchronously, using the glib thread-pool.
For that to work, we also need to switch the network-namespace of the
thread-pool thread. We want to use NMPNetns for that. Hence it's better
to have NMPNetns thread-safe, instead of coming up with a duplicate
implementation. But NMPNetns may want to log, so we also need nm-logging
thread-safe.

In general, code under "shared/nm-utils" and nm-logging should be usable
from multiple threads. It's simpler to make this code thread-safe than
re-implementing it. Also, it's a bad limitation to be unable to log
from other threads. If there is an error, the best we can often do is to
log about it.

Make nm-logging thread-safe. Actually, we only need to be able to log
from multiple threads. We don't need to setup or configure logging from
multiple threads. This restriction allows us to access logging from the
main-thread without any thread-synchronization (because all changes in
the logging setup are also done from the main-thread).

So, while logging from other threads requires a mutex, logging from the
main-thread is lock-free.

Instead of having two functions nm_logging_set_syslog_identifier()
and nm_logging_set_prefix(), merge them.

They must both be called at earliest point and together. No point
in giving them the appearance that they could be called any time.

Previously, _nm_logging_clear_platform_logging_cache was an extern variable,
and NMLinuxPlatform would set it to a function pointer at certain points.

That's unnecessary complex, also when trying to make nm-logging thread-safe,
it's just more global variables that need to be considered. Don't do it
that way, but just link in a regular function.

In core ("src/"), we use "nm-logging.h" for all logging. This dispatches
for logging to syslog, glog or systemd-journald.

If we want to log from a shared component under "shared/", we need to
use a common logging function. Add "nm-utils/nm-logging-fwd.h" for
forward declaring the used logging mechaism.

The shared library will still need to link with "src/nm-logging.c"
or an alternative implementation, depending on whether it is used
inside core or not.

Correct the spelling across the *entire* tree, including translations,
comments, etc. It's easier that way.

Even the places where it's not exposed to the user, such as tests, so
that we learn how is it spelled correctly.

"nm-logging.c" now no longer depends on anything particularly special
from NM core.

Using '#ifdef' is generally error prone. It's better to always define
a define and check for it explicitly. This way, the compiler can issue
a warning if the define does not exist.

Also, note how meson would always define NM_MORE_LOGGING, possibly to
"0". That means, for meson, we unintentionally always enabled more
logging because the define was always present.

Fix that.

"debug" was documentation in `man NetworkManager.conf` as a valid
logging backend. However, it was completely ignored by
nm_logging_syslog_openlog().
In fact, it makes not sense. Passing debug = TRUE to
nm_logging_syslog_openlog(), means that all messages will be
printed to stderr in addition to syslog/journal. However, when
NetworkManager is daemonizing, stderr is closed.
Whether NetworkManager is daemonizing depends entirely on command
line options --no-daemon and --debug. Hence, the logging backend "debug"
from the configuration file either conflicts or is redundant.

Also, adjust logging backend description in `man NetworkManager.conf`.

Also, log a warning about invalid/unsupported logging backend.

(cherry picked from commit 2ccf6168dc1c54fde350ec669b777c29d566fb4a)

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

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