platform: make global logging cache for NMPlatform's sysctl values thread-safe

We have a cache for sysctl values, so that we can log changes and
previous values.

When resetting the log level, we prune that cache, which is done by
_nm_logging_clear_platform_logging_cache(). That function is called
by nm_logging_setup(), which is guaranteed to only happen on the main
thread.

NMPlatform in general is not thread safe (meaning, that the same NMPlatform
instance cannot be used by multiple threads at the same time). There is however
a reasonable aim that you could use different NMPlatform instances on their
own threads.

That currently doesn't work, mainly due to nm-logging which always must
be done from the main thread -- unless we would set NM_THREAD_SAFE_ON_MAIN_THREAD
in all of NMPlatform (which would be too expensive for something we
don't actually need). That means also the sysctl getter must only be
called on the main thread an all was good already.

Still, we could have NMPlatform usable from multiple thread by setting
NM_THREAD_SAFE_ON_MAIN_THREAD. As we are almost there to have the code
thread-safe, make accessing the sysctl value cache thread-safe (even if
we currently don't actually access it from multiple thread).

src/libnm-log-core/nm-logging.c

@@ -402,7 +402,10 @@ nm_logging_setup(const char *level, const char *domains, char **bad_domains, GEr
     if (had_platform_debug && !_nm_logging_enabled_lockfree(LOGL_DEBUG, LOGD_PLATFORM)) {
         /* when debug logging is enabled, platform will cache all access to
          * sysctl. When the user disables debug-logging, we want to clear that
-         * cache right away. */
+         * cache right away.
+         *
+         * It's important that we call this without having a lock on "log", because
+         * otherwise we might deadlock. */
         _nm_logging_clear_platform_logging_cache();
     }