tools lib traceevent: Changed return logic of tep_register_event_handler() API

In order to make libtraceevent into a proper library, its API
should be straightforward.

The tep_register_event_handler() functions returns -1 in case it
successfully registers the new event handler. Such return code is used
by the other library APIs in case of an error.

To unify the return logic of tep_register_event_handler() with the other
APIs, this patch introduces enum tep_reg_handler, which is used by this
function as return value, to handle all possible successful return
cases.

Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20181201040852.628034497@goodmis.org


Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>