Clear input/output buffers on macOS upon TTY open

I don't understand why this works, but it resolves problematic behavior
(#49) on macOS. Bisecting history, I found commit
3d6c1b2, which removed flushing the IO
buffers for a TTY, was problematic on macOS. However, the test
`test_osx_pty_pair` which tests pseudo-ttys with baud_rate 0 on macOS
(introduced in b54264b) fails if I
simply reintroduced flushing. Therefore, this change now re-introduces
flushing the IO buffers upon opening a TTY, but only on macOS and only
when baud-rate is non-zero.

Thanks to @berkus for the original work.

src/posix/tty.rs

@@ -156,6 +156,11 @@ impl TTYPort {
             ));
         };
 
+        #[cfg(any(target_os = "ios", target_os = "macos"))]
+        if builder.baud_rate > 0 {
+            unsafe { libc::tcflush(fd.0, libc::TCIOFLUSH) };
+        }
+
         // clear O_NONBLOCK flag
         fcntl(fd.0, F_SETFL(nix::fcntl::OFlag::empty()))?;