Kernel: Add basic support for O_NONBLOCK (only for tty)

kernel/kernel/OpenFileDescriptorSet.cpp

@@ -234,6 +234,8 @@ namespace Kernel
 	{
 		TRY(validate_fd(fd));
 		auto& open_file = m_open_files[fd];
+		if ((open_file->flags & O_NONBLOCK) && !open_file->inode->has_data())
+			return 0;
 		size_t nread = TRY(open_file->inode->read(open_file->offset, buffer, count));
 		open_file->offset += nread;
 		return nread;

kernel/kernel/Terminal/TTY.cpp

@@ -679,6 +679,12 @@ flush:
 		return count;
 	}
 
+	bool TTY::has_data() const
+	{
+		LockGuard _(m_lock);
+		return m_output.flush;
+	}
+
 	void TTY::putchar_current(uint8_t ch)
 	{
 		ASSERT(s_tty);