can-utils: canfdtest, a full duplex test to find out-of-order messages

can-utils: canfdtest, a full duplex test to find out-of-order messages

This patch is a pepped up version of Valdislav's canecho_gen and
canecho_dut test programs, which have prooved to be useful for
detecting out-of-order message transmisson and reception. Here
is a list of the changes and improvements:

- Both programs have been merged into on test program named
  canfdtest. Message generation can be selected via the command
  line option '-g'.
- The test loop count can be specified.
- A low and high verbosity level has been added.
- send/recv is used instead of write/read.
- The return code of send/recv is checked properly.
- Use Linux coding style.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>