BUG/MEDIUM: mworker: don't close stdio several time

BUG/MEDIUM: mworker: don't close stdio several time

This patch makes sure that a frontend socket that gets created after
initialization won't be closed when the master gets re-executed.

When used in daemon mode, the master-worker is closing the FDs 0, 1, 2
after the fork of the children.

When the master was reloading, those FDs were assigned again during the
parsing of the configuration (probably for some listeners), and the
workers were closing them thinking it was the stdio.

This patch must be backported to 1.8.