Refactor cryptcheck() to allow multiple list-files and stdin

Previously, it was not possible to use

sha1sum test.c | sha1sum -c

because the program would not differenciate between an empty
argument and a non-specified argument.
Moreover, why not allow this?

sha1sum -c hashlist1 hashlist2

Digging deeper I found that using function pointers and a
modification in the crypt-backend might simplify the program
a lot by passing the argument-list to both cryptmain and
cryptcheck.
Allowing more than one list-file to be specified is also
consistent with what the other implementations support,
so we not only have simpler code, we also do not silently
break if there's a script around passing multiple files to
check.

md5sum.c

@@ -24,20 +24,16 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
+	int (*cryptfunc)(int, char **, struct crypt_ops *, uint8_t *, size_t) = cryptmain;
 	uint8_t md[MD5_DIGEST_LENGTH];
-	char *checkfile = NULL;
-	int cflag = 0;
 
 	ARGBEGIN {
 	case 'c':
-		cflag = 1;
-		checkfile = ARGF();
+		cryptfunc = cryptcheck;
 		break;
 	default:
 		usage();
 	} ARGEND;
 
-	if (cflag)
-		return cryptcheck(checkfile, argc, argv, &md5_ops, md, sizeof(md));
-	return cryptmain(argc, argv, &md5_ops, md, sizeof(md));
+	return cryptfunc(argc, argv, &md5_ops, md, sizeof(md));
 }