When called with -e, epoll is used instead of poll. The poller does
very little in this code (just checks for any event without waiting) but
that's sufficient to see return values.

Some interleaved dumps were hard to follow. By indicating which side
we're talking about it's easier. E.g:

  $ dev/poll/poll -v -s pol,clo -c pol
  #### BEGIN ####
  cmd #1 stp #0: con(c=4): ret=0
  cmd #1 stp #0: acc(l=3): ret=5
  cmd #1 stp #1: pol(s=5): ret=1 ev=0x4 (OUT)
  cmd #1 stp #2: clo(s=5): ret=0
  cmd #2 stp #1: pol(c=4): ret=1 ev=0x2005 (IN OUT RDHUP)

This doesn't bring anything and makes the output less readable. Let's
just keep the action name.

Now the connect() step becomes an action. It's still implicit before
any -c/-s but it allows the listener to close() before connect()
happens, showing the polling status for this condition:

  $ dev/poll/poll -v -l clo -c pol
  #### BEGIN ####
  cmd #1 stp #1: do_clo(3): ret=0
  cmd #2 stp #0: do_con(4): ret=-1 (Connection refused)
  cmd #2 stp #1: do_pol(4): ret=1 ev=0x14 (OUT HUP)
  #### END ####

which differs from a case where the server closes the just accepted
connection:

  $ dev/poll/poll -v -s clo -c pol
  #### BEGIN ####
  cmd #1 stp #0: do_con(4): ret=0
  cmd #1 stp #0: do_acc(3): ret=5
  cmd #1 stp #1: do_clo(5): ret=0
  cmd #2 stp #1: do_pol(4): ret=1 ev=0x2005 (IN OUT RDHUP)
  #### END ####

It's interesting to see OUT+HUP since HUP indicates that both directions
were closed, hence nothing may be written now, thus OUT just wants the
write handler to be notified.

The following directories were moved from contrib/ to dev/ to make their
use case a bit clearer. In short, only developers are expected to ever
go there. The makefile was updated to build and clean from these ones.

base64/  flags/  hpack/  plug_qdisc/  poll/  tcploop/  trace/