tools lib api fs tracing_path: Introduce get/put_events_file() helpers

To make reading events files a tad more compact than with
get_tracing_files("events/foo").

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-do6xgtwpmfl8zjs1euxsd2du@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>