MEDIUM: mworker: try to guess the next stats socket to use with -x

MEDIUM: mworker: try to guess the next stats socket to use with -x

In master worker mode, you can't specify the stats socket where you get
your listeners FDs on a reload, because the command line of the re-exec
is launched by the master.

To solve the problem, when -x is found on the command line, its
parameter is rewritten on a reexec with the first stats socket with the
capability to send sockets. It tries to reuse the original parameter if
it has this capability.