journal: Don't discard kmsg messages coming from journald itself

Previously, we discarded any kmsg messages coming from journald
itself to avoid infinite loops where potentially the processing
of a kmsg message causes journald to log one or more messages to
kmsg which then get read again by the kmsg handler, ...

However, if we completely disable logging whenever we're processing
a kmsg message coming from journald itself, we also prevent any
infinite loops as we can be sure that journald won't accidentally
generate logging messages while processing a kmsg log message.

This change allows us to store all journald logs generated during
the processing of log messages from other services in the system
journal. Previously these could only be found in kmsg which has
low retention, can't be queried using journalctl and whose logs
don't survive reboots.

src/basic/log.h

@@ -27,6 +27,10 @@ typedef enum LogTarget{
         _LOG_TARGET_INVALID = -EINVAL,
 } LogTarget;
 
+/* This log level disables logging completely. It can only be passed to log_set_max_level() and cannot be
+ * used a regular log level. */
+#define LOG_NULL (LOG_EMERG - 1)
+
 /* Note to readers: << and >> have lower precedence than & and | */
 #define SYNTHETIC_ERRNO(num)                (1 << 30 | (num))
 #define IS_SYNTHETIC_ERRNO(val)             ((val) >> 30 & 1)