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/basic/process-util.c

@@ -1381,8 +1381,8 @@ void valgrind_summary_hack(void) {
                 if (pid < 0)
                         log_struct_errno(
                                 LOG_EMERG, errno,
-                                "MESSAGE_ID=" SD_MESSAGE_VALGRIND_HELPER_FORK_STR,
-                                LOG_MESSAGE( "Failed to fork off valgrind helper: %m"));
+                                LOG_MESSAGE_ID(SD_MESSAGE_VALGRIND_HELPER_FORK_STR),
+                                LOG_MESSAGE("Failed to fork off valgrind helper: %m"));
                 else if (pid == 0)
                         exit(EXIT_SUCCESS);
                 else {