Support reading checksums from stdin

sha1sum.c

@@ -1,4 +1,5 @@
 /* See LICENSE file for copyright and license details. */
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
@@ -25,16 +26,18 @@ main(int argc, char *argv[])
 {
 	uint8_t md[SHA1_DIGEST_LENGTH];
 	char *checkfile = NULL;
+	bool cflag = false;
 
 	ARGBEGIN {
 	case 'c':
-		checkfile = EARGF(usage());
+		cflag = true;
+		checkfile = ARGF();
 		break;
 	default:
 		usage();
 	} ARGEND;
 
-	if(checkfile)
+	if(cflag)
 		return cryptcheck(checkfile, argc, argv, &sha1_ops, md, sizeof(md));
 	return cryptmain(argc, argv, &sha1_ops, md, sizeof(md));
 }