tree-wide: use LOG_MESSAGE() where possible

Also break some long lines for more uniform formatting. No functional change.

I went over all log_struct, log_struct_errno, log_unit_struct,
log_unit_struct_errno calls, and they seem fine.

src/basic/log.h

@@ -298,7 +298,8 @@ int log_emergency_level(void);
 
 bool log_on_console(void) _pure_;
 
-/* Helper to prepare various field for structured logging */
+/* Helper to wrap the main message in structured logging. The macro doesn't do much,
+ * except to provide visual grouping of the format string and its arguments. */
 #define LOG_MESSAGE(fmt, ...) "MESSAGE=" fmt, ##__VA_ARGS__
 
 void log_received_signal(int level, const struct signalfd_siginfo *si);