PR26019 stapbpf: don't continue startup if begin probe already exited

PR26019 stapbpf: don't continue startup if begin probe already exited

if a bpf begin probe sets exit status, the main thread will skip the
pause() call and immediately deallocate global data structures. Then
the perf_event_loop thread will run on the corrupted data and
occasionally fail. Example result is an assertion failure due
count_active_cpus() running on corrupted data.

Could change the main thread to join() instead of detach(), but then
all threads must be modified to listen for exit status including exit
status from begin thread. TODO Consider doing so later.

I think if the begin probe was running, it wasn't correct to start the
perf_event_loop (or PERF_EVENT_IOC_ENABLE, or any of the procfs
threads) in the first place, as those things logically happen after
the begin probe has already exited. Simply clean up and exit.