cope with no argv0, use libio

src/simexec.c

@@ -1,16 +1,18 @@
 #include <stddef.h> /* NULL */
 #include <sysexits.h> /* EX_USAGE */
 #include <unistd.h> /* execv(3), write(2) */
-#include "noargvzero.h"
+#include "libio.h" /* fdprint(3) */
+
+static char *program_name = "simexec";
 
 int main(int argc, char *argv[]){
 	int i;
-	NOARGVZERO(argv);
 
 	if(argc < 2){
 		write(2, "Usage: ", 7);
-		for(i = 0; argv[0][i] != '\0'; ++i);
-		write(2, argv[0], i);
+		if(argv[0] == NULL)
+			argv[0] = program_name;
+		fdprint(2, argv[0]);
 		write(2, " [pathname], [argv...]\n", 23);
 		return EX_USAGE;
 	}