Fix type for write(2) return variable.

A little fix in xwrite().

>From 3727d2e334 Mon Sep 17 00:00:00 2001
From: Quentin Rameau <quinq@fifth.space>
Date: Fri, 24 Jul 2015 11:40:46 +0200
Subject: [PATCH] Fix type for write(2) return variable.

The allocated lengh of s fits into an integer so we can safely use
ssize_t here.

st.c

@@ -560,7 +560,8 @@ static int frclen = 0;
 ssize_t
 xwrite(int fd, const char *s, size_t len)
 {
-	size_t aux = len, r;
+	size_t aux = len;
+	ssize_t r;
 
 	while (len > 0) {
 		r = write(fd, s, len);