Switch concat() to use fread() and fwrite()

We should never mix FILE I/O with raw I/O.  Going from raw I/O
to FILE I/O is fine but doing the opposite is extremely tricky and
only works under certain conditions (unbuffered stream + no call
to ungetc()).

libutil/concat.c

@@ -1,6 +1,5 @@
 /* See LICENSE file for copyright and license details. */
 #include <stdio.h>
-#include <unistd.h>
 
 #include "../text.h"
 #include "../util.h"
@@ -9,12 +8,14 @@ void
 concat(FILE *fp1, const char *s1, FILE *fp2, const char *s2)
 {
 	char buf[BUFSIZ];
-	ssize_t n;
+	size_t n;
 
-	while ((n = read(fileno(fp1), buf, sizeof buf)) > 0) {
-		if (write(fileno(fp2), buf, n) != n)
+	while ((n = fread(buf, 1, sizeof(buf), fp1)) > 0) {
+		if (fwrite(buf, 1, n, fp2) != n)
 			eprintf("%s: write error:", s2);
+		if (feof(fp1))
+			break;
 	}
-	if (n < 0)
+	if (ferror(fp1))
 		eprintf("%s: read error:", s1);
 }