Extract the FDs and flags from poll and ppoll

Extract the FDs and flags from poll and ppoll

Instead of printing the pointer address of the poll set, extract all the
FDs and flags from the poll set. For now, we only output the
standardized set of events to limit the verbosity of the output, we also
extract the raw value. When we switch to CTF2 we will be able to hide
unset fields and then we will extract all the fields.

Here is an example of output with one FD:
syscall_entry_poll: {
  timeout_msecs = -1, nfds = 1, fds_length = 1,
  fds = [
    [0] = { fd = 4, raw_events = 0x5, events = { POLLIN = 1, POLLPRI = 0,
            POLLOUT = 1, POLLERR = 0, POLLHUP = 0, padding = 0 } } ]
}

syscall_exit_poll: {
  ret = 1, nfds = 1, fds_length = 1,
  fds = [ [0] = { fd = 4, raw_events = 0x4, events = { POLLIN = 0,
  POLLPRI = 0, POLLOUT = 1, POLLERR = 0, POLLHUP = 0, padding = 0 } } ] }

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>