Simplify return & fshut() logic

Get rid of the !!()-constructs and use ret where available (or introduce it).

In some cases, there would be an "abort" on the first fshut-error, but we want
to close all files and report all warnings and then quit, not just the warning
for the first file.

sed.c

@@ -1682,7 +1682,7 @@ int
 main(int argc, char *argv[])
 {
 	char *arg;
-	int script = 0;
+	int ret = 0, script = 0;
 
 	ARGBEGIN {
 	case 'n':
@@ -1717,5 +1717,7 @@ main(int argc, char *argv[])
 	files = argv;
 	run();
 
-	return !!(fshut(stdin, "<stdin>") + fshut(stdout, "<stdout>"));
+	ret |= fshut(stdin, "<stdin>") | fshut(stdout, "<stdout>");
+
+	return ret;
 }