libutil: Add writeall utility function

writeall makes successive write calls to write an entire buffer to the
output file descriptor. It returns the number of bytes written, or -1 on
the first error.

libutil/writeall.c

@@ -0,0 +1,21 @@
+/* See LICENSE file for copyright and license details. */
+#include <unistd.h>
+
+#include "../util.h"
+
+ssize_t
+writeall(int fd, const void *buf, size_t len)
+{
+	const char *p = buf;
+	ssize_t n;
+
+	while (len) {
+		n = write(fd, p, len);
+		if (n <= 0)
+			return n;
+		p += n;
+		len -= n;
+	}
+
+	return p - (const char *)buf;
+}