Instead of manually walking the stack and printing each frame to the
trace log, we can use a BPF_STACK_TRACE table to store the stack traces,
and print to the trace log their IDs every time they are traced.

The output of the tool has been slightly modified: we no longer prefix
each line of the stack trace with the timestamp and the other headers,
since the whole stack trace is fetched in one go from the table and the
information would be highly redundant.

The man page and the examples have been updated to reflect the new
output.