perf header: Propagate feature section processing errors

perf header: Propagate feature section processing errors

perf_session__read_header() discards the return value from
perf_header__process_sections(), so any error from a feature
section processor (process_nrcpus, process_compressed, etc.)
is silently ignored and the session opens as if nothing went
wrong.

This defeats the validation added by subsequent commits in this
series: a crafted perf.data that fails a feature section check
would still be processed with partially-initialized state.

Check the return value and fail the session if any feature
section processor returns an error.

For truncated files (data.size == 0, i.e. recording was
interrupted before the header was finalized), skip feature
section processing entirely and clear the feature bitmap so
tools use their "feature not present" fallbacks instead of
accessing uninitialized env fields.

Change the feature processor stubs for optional libraries
(libtraceevent, libbpf) from returning -1 to returning 0,
so that perf.data files containing these features can still be
opened on builds without the optional library — the feature is
simply skipped rather than causing a fatal error.

Also propagate evlist__prepare_tracepoint_events() failure as
-ENOMEM, since the function can fail due to strdup() allocation
failure inside evsel__prepare_tracepoint_event().

Fixes: 1c0b04d12ae9 ("perf tools: Add perf_session__read_header function")
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude:claude-opus-4.6-1m
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>