perf trace: Make evlist__set_evsel_handler() affect just entries without a handler

Renaming it to evlist__set_default_evsel_handler(), to better reflect
what we want to do, which is to set a default handler for events we
still haven't set a custom handler, like the ones for "msr:write_msr",
etc that are coming soon.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-e1bit7upnpmtsayh8039kfuw@git.kernel.org


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