dj(1), mm(1), npc(1), scrut(1), str(1): consistent argv[0] handling

src/npc.c

@@ -23,6 +23,8 @@
 #include <unistd.h> /* getopt(3) */
 #include <sysexits.h> /* EX_OK, EX_USAGE */
 
+static char *program_name = "dj";
+
 int usage(char *s) {
 	fprintf(stderr, "Usage: %s [-et]\n", s);
 	return EX_USAGE;
@@ -32,18 +34,19 @@ int main(int argc, char *argv[]) {
 	int c;
 	char showend = 0; /* print a dollar sign before each newline */
 	char showtab = 0; /* prints tab characters in caret notation */
+	char *s = (argv[0] == NULL ? program_name : argv[0]);
 
 	if (argc > 0) {
 		while ((c = getopt(argc, argv, "et")) != -1) {
 			switch (c){
 				case 'e': showend = 1; break;
 				case 't': showtab = 1; break;
-				default:  return usage(argv[0]);
+				default:  return usage(s);
 			}
 		}
 	}
 
-	if (argc > optind) { return usage(argv[0]); }
+	if (argc > optind) { return usage(s); }
 
 	while ((c = getc(stdin)) != EOF) {
 		if ((c & 0x80) != 0) { fputs("M-", stdout); }