barectf-tracepoint.h: improve quality

barectf-tracepoint.h: improve quality

Changes:

* Reflow header comment to fit 72 columns.

* Capitalize comments.

* Use updated `barectf.h` definition names (`_BARECTF_IDENTIFIER_PREFIX`
  and `_BARECTF_DEFAULT_DATA_STREAM_TYPE_NAME`).

* Check for `BARECTF_TRACEPOINT_DATA_STREAM_TYPE_NAME` as an alternative
  to `BARECTF_TRACEPOINT_STREAM` (for consistency).

* Rework error messages.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>