signal: add marco _NSIG
#define SIGRTMIN MIN_SIGNO /* First real time signal */
#define SIGRTMAX MAX_SIGNO /* Last real time signal */
+#define _NSIG (MAX_SIGNO + 1) /* Biggest signal number + 1 */
/* NuttX does not support all standard signal actions. NuttX supports what
* are referred to as "real time" signals. The default action of all NuttX
