BUG/MEDIUM: cli: make "show cli sockets" really yield

BUG/MEDIUM: cli: make "show cli sockets" really yield

This command was introduced in 1.8 with commit eceddf722 ("MEDIUM: cli:
'show cli sockets' list the CLI sockets") but its yielding doesn't work.
Each time it enters, it restarts from the last bind_conf but enumerates
all listening sockets again, thus it loops forever. The risk that it
happens in field is low but it easily triggers on port ranges after
400-500 sockets depending on the length of their addresses:

  global
     stats socket /tmp/sock1 level admin
     stats socket 192.168.8.176:30000-31000 level operator

  $ socat /tmp/sock1 - <<< "show cli sockets"
  (...)
  ipv4@192.168.8.176:30426 operator all
  ipv4@192.168.8.176:30427 operator all
  ipv4@192.168.8.176:30428 operator all
  ipv4@192.168.8.176:30000 operator all
  ipv4@192.168.8.176:30001 operator all
  ipv4@192.168.8.176:30002 operator all
  ^C

This patch adds the minimally needed restart point for the listener so
that it can easily be backported. Some more cleanup is needed though.