BUG/MEDIUM: mworker: create the mcli_reload socketpairs in case of upgrade

BUG/MEDIUM: mworker: create the mcli_reload socketpairs in case of upgrade

In ticket #1956, it was reported that an upgrade from 2.6 to 2.7 via a
reload would stop the master process.

When upgrading the binary, the new process is considered reexec and does
not try to creates the socketpair for the mcli_reload listener, then
tries to bind on -1 since the socket doesn't exit. The failure provokes
an exit() of the master.

This patch fixes the issue by trying to create the mcli_reload sockets
only when they don't exist, instead of creating them at first start.
This way we also avoid possible fd leak since we always try to use the
existing FDs first.

Must be backported in 2.7.