Bionic lacks ttyname_r; provide a workaround

Signed-off-by: Matt Whitlock <busybox@mattwhitlock.name>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/platform.c

@@ -194,3 +194,22 @@ ssize_t FAST_FUNC getline(char **lineptr, size_t *n, FILE *stream)
 	return len;
 }
 #endif
+
+#ifndef HAVE_TTYNAME_R
+int ttyname_r(int fd, char *buf, size_t buflen)
+{
+	int r;
+	char path[sizeof("/proc/self/fd/%d") + sizeof(int)*3];
+
+	if (!isatty(fd))
+		return errno == EINVAL ? ENOTTY : errno;
+	sprintf(path, "/proc/self/fd/%d", fd);
+	r = readlink(path, buf, buflen);
+	if (r < 0)
+		return errno;
+	if (r >= buflen)
+		return ERANGE;
+	buf[r] = '\0';
+	return 0;
+}
+#endif