stream: autodetect direction

Previously, we required the user to specify the expected read/write
flags for a pipe or tty. But we've already been asking the OS to tell us
what they actually are (fcntl F_GETFL), so we can hopefully just use
that information directly.

Fixes: libuv/libuv#1936
PR-URL: libuv/libuv#1964
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

docs/code/tty-gravity/main.c

@@ -32,7 +32,7 @@ void update(uv_timer_t *req) {
 int main() {
     loop = uv_default_loop();
 
-    uv_tty_init(loop, &tty, 1, 0);
+    uv_tty_init(loop, &tty, STDOUT_FILENO, 0);
     uv_tty_set_mode(&tty, 0);
 
     if (uv_tty_get_winsize(&tty, &width, &height)) {

docs/code/tty/main.c

@@ -8,7 +8,7 @@ uv_tty_t tty;
 int main() {
     loop = uv_default_loop();
 
-    uv_tty_init(loop, &tty, 1, 0);
+    uv_tty_init(loop, &tty, STDOUT_FILENO, 0);
     uv_tty_set_mode(&tty, UV_TTY_MODE_NORMAL);
 
     if (uv_guess_handle(1) == UV_TTY) {