MINOR: global: Add an option to get the old listening sockets.

MINOR: global: Add an option to get the old listening sockets.

Add the "-x" flag, that takes a path to a unix socket as an argument. If
used, haproxy will connect to the socket, and asks to get all the
listening sockets from the old process. Any failure is fatal.
This is needed to get seamless reloads on linux.