concat: Use plain read/write instead of buffered stdio

If we are just copying data from one file to another, we don't need to
fill a complete buffer, just read a chunk at a time, and write it to the
output.

libutil/concat.c

@@ -1,19 +1,23 @@
 /* See LICENSE file for copyright and license details. */
-#include <stdio.h>
+#include <unistd.h>
 
-#include "../text.h"
 #include "../util.h"
 
-void
-concat(FILE *fp1, const char *s1, FILE *fp2, const char *s2)
+int
+concat(int f1, const char *s1, int f2, const char *s2)
 {
 	char buf[BUFSIZ];
-	size_t n;
+	ssize_t n;
 
-	while ((n = fread(buf, 1, sizeof(buf), fp1))) {
-		fwrite(buf, 1, n, fp2);
-
-		if (feof(fp1) || ferror(fp1) || ferror(fp2))
-			break;
+	while ((n = read(f1, buf, sizeof(buf))) > 0) {
+		if (writeall(f2, buf, n) < 0) {
+			weprintf("write %s:", s2);
+			return -2;
+		}
 	}
+	if (n < 0) {
+		weprintf("read %s:", s1);
+		return -1;
+	}
+	return 0;
 }