AP_HAL_Linux: Add support for in-kernel SPI CS handling

Add support for /dev/spidev<bus>.<cs> in-kernel SPI CS handling, and convert
existing SPI devices over.

 * Add a new member _subdev to the LinuxSPIDeviceDriver class, and initialiser
   in LinuxSPIDevice constructor.  Use subdev 0 for GPIO-managed CS
 * Extend the _cs_pin to signed 16 bits and define SPI_CS_KERNEL (-1) for
   kernel-managed CS
 * Move to per-device file descriptors (required for in-kernel CS)
 * Extend spidev filenames to include bus and subdevice numbers, possibly
   longer than 1 digit each
 * Allow support for platforms enumerating /dev/spidevNNN from non-zero bases
 * Convert existing users over to the new API

Signed-off-by: John Williams <john@whelanwilliams.net>