cat: fix u flag

Our libutil concat() always uses a buffer for input wich is obviously
not compatible with unbuffered io. Here is a local uconcat() which
naively copies input to stdout char by char.

cat.c

@@ -6,6 +6,16 @@
 #include "text.h"
 #include "util.h"
 
+static void
+uconcat(FILE *fp1, const char *s1, FILE *fp2, const char *s2)
+{
+	int c;
+
+	setbuf(fp2, NULL);
+	while ((c = getc(fp1)) != EOF)
+		putc(c, fp2);
+}
+
 static void
 usage(void)
 {
@@ -17,17 +27,18 @@ main(int argc, char *argv[])
 {
 	FILE *fp;
 	int ret = 0;
+	void (*cat)(FILE *, const char *, FILE *, const char *) = &concat;
 
 	ARGBEGIN {
 	case 'u':
-		setbuf(stdout, NULL);
+		cat = &uconcat;
 		break;
 	default:
 		usage();
 	} ARGEND
 
 	if (!argc) {
-		concat(stdin, "<stdin>", stdout, "<stdout>");
+		cat(stdin, "<stdin>", stdout, "<stdout>");
 	} else {
 		for (; *argv; argc--, argv++) {
 			if (!strcmp(*argv, "-")) {
@@ -38,7 +49,7 @@ main(int argc, char *argv[])
 				ret = 1;
 				continue;
 			}
-			concat(fp, *argv, stdout, "<stdout>");
+			cat(fp, *argv, stdout, "<stdout>");
 			if (fp != stdin && fshut(fp, *argv))
 				ret = 1;
 		}