drivers/serial/pty.c, serial.c, usbdev/cdcacm.c, include/nuttx/fs/ioctl.h: ...

drivers/serial/pty.c, serial.c, usbdev/cdcacm.c, include/nuttx/fs/ioctl.h:  Fix FIONWRITE and add FIONSPACE.  All implementations of FIONWRITE were wrong.  FIONWRITE should return the number of bytes waiting in the outgoing send queue, not the free space.  Rather, FIONSPACE should return the free space in the send queue.