tree-wide: check more log message format in log_struct() and friends

This introduce LOG_ITEM() macro that checks arbitrary formats in
log_struct().
Then, drop _printf_ attribute from log_struct_internal(), as it does not
help so much, and compiler checked only the first format string.

Hopefully, this silences false-positive warnings by Coverity.

src/core/execute.c

@@ -463,7 +463,7 @@ static void log_command_line(Unit *unit, const char *msg, const char *executable
         _cleanup_free_ char *cmdline = quote_command_line(argv, SHELL_ESCAPE_EMPTY);
 
         log_unit_struct(unit, LOG_DEBUG,
-                        "EXECUTABLE=%s", executable,
+                        LOG_ITEM("EXECUTABLE=%s", executable),
                         LOG_UNIT_MESSAGE(unit, "%s: %s", msg, strnull(cmdline)),
                         LOG_UNIT_INVOCATION_ID(unit));
 }