MINOR: cli: add 'expose-fd listeners' to pass listeners FDs

MINOR: cli: add 'expose-fd listeners' to pass listeners FDs

This patch changes the stats socket rights for allowing the sending of
listening sockets.

The previous behavior was to allow any unix stats socket with admin
level to send sockets. It's not possible anymore, you have to set this
option to activate the socket sending.

Example:
   stats socket /var/run/haproxy4.sock mode 666 expose-fd listeners level user process 4