DEV: poll: indicate the FD's side in front of its value

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)
1 file changed