bin: compile lttng-relayd as a C++

Same as the previous commit, but change lttng-relayd to be a C++
program.  In addition to what was mentioned in previous commits:

 - Test test_relayd_backward_compat_group_by_session uses an object file
   from the relayd binary, so it's simpler to change this test to be C++
   as well.

 - There are some declarations related to fs_handle in fd-tracker.h.
   Remove them, as they are already in fs-handle.h.

Change-Id: I72c5fdecd2e82c30633563a47bd641af1dcfa29c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>