Expose the CAN_BCM SocketCAN constants used in the bcm_msg_head struct
flags (provided by <linux/can/bcm.h>) under the socket library.

This adds the following constants with a CAN_BCM prefix:

  * SETTIMER
  * STARTTIMER
  * TX_COUNTEVT
  * TX_ANNOUNCE
  * TX_CP_CAN_ID
  * RX_FILTER_ID
  * RX_CHECK_DLC
  * RX_NO_AUTOTIMER
  * RX_ANNOUNCE_RESUME
  * TX_RESET_MULTI_IDX
  * RX_RTR_FRAME
  * CAN_FD_FRAME

The CAN_FD_FRAME flag was introduced in the 4.8 kernel, while the other
ones were present since SocketCAN drivers were mainlined in 2.6.25. As
such, it is probably unnecessary to guard against these constants being
missing.

The CAN_BCM_CAN_FD_FRAME constant mirrors the availability in the Linux
kernel. This constant was introduced in Linux 4.8 kernels.

Use support.requires_linux_version to guard BCM flag CAN_FD_FRAME,
which is only present on Linux >= 4.8.

This constant doesn't exist on Ubuntu 16.04 LTS, since linux-libc-dev
ships 4.4.x headers from the repos, while the feature was added in the
4.8.x kernel series. However, Ubuntu 16.04 supports the 4.15.x kernel
series, but since linux-libc-dev isn't updated past 4.4.x headers, there
is no guarantee that /usr/include/linux/can/bcm.h contains the proper
constant for the userspace API, despite having a supported kernel.

Fix the sphinx warning message that is causing the CI failure:

    Content block expected for the "note" directive; none found.

This is done by increasing the indentation to place the text in a block.