CONTRIB: debug: add new program "poll" to test poll() events

CONTRIB: debug: add new program "poll" to test poll() events

This simple program prepares a TCP connection between two ends and
allows to perform various operations on them such as send, recv, poll,
shutdown, close, reset, etc. It takes care of remaining particularly
silent to help inspection via strace, though it can also be verbose
and report status, errno, and poll events. It delays acceptation of
the incoming server-side connection so that it's even possible to
test the poll status on a listener with a pending connection, or
to close the connection without accepting it and inspect the effect
on the client.

Actions are executed in the command line order as they are parsed,
they may be grouped using commas when they are performed on the same
socket.

Example showing a successful recv() of pending data before a pending error:
   $ ./poll -v -l pol,acc,pol -c snd,shw -s pol,rcv,pol,rcv,pol,snd,lin,clo -c pol,rcv,pol,rcv,pol

   #### BEGIN ####
   cmd #1 stp #1: do_pol(3): ret=1 ev=0x1 (IN)
   cmd #1 stp #2: do_acc(3): ret=5
   cmd #1 stp #3: do_pol(3): ret=0 ev=0
   cmd #2 stp #1: do_snd(4): ret=3
   cmd #2 stp #2: do_shw(4): ret=0
   cmd #3 stp #1: do_pol(5): ret=1 ev=0x2005 (IN OUT RDHUP)
   cmd #3 stp #2: do_rcv(5): ret=3
   cmd #3 stp #3: do_pol(5): ret=1 ev=0x2005 (IN OUT RDHUP)
   cmd #3 stp #4: do_rcv(5): ret=0
   cmd #3 stp #5: do_pol(5): ret=1 ev=0x2005 (IN OUT RDHUP)
   cmd #3 stp #6: do_snd(5): ret=3
   cmd #3 stp #7: do_lin(5): ret=0
   cmd #3 stp #8: do_clo(5): ret=0
   cmd #4 stp #1: do_pol(4): ret=1 ev=0x201d (IN OUT ERR HUP RDHUP)
   cmd #4 stp #2: do_rcv(4): ret=3
   cmd #4 stp #3: do_pol(4): ret=1 ev=0x201d (IN OUT ERR HUP RDHUP)
   cmd #4 stp #4: do_rcv(4): ret=-1 (Connection reset by peer)
   cmd #4 stp #5: do_pol(4): ret=1 ev=0x2015 (IN OUT HUP RDHUP)
   #### END ####