BUG/MEDIUM: mworker: cleanup the listeners when reexecuting

BUG/MEDIUM: mworker: cleanup the listeners when reexecuting

Previously, the cleanup of the listeners was done in mworker_loop(),
which was called once the configuration file was parsed. HAProxy was
switching in wait mode when the configuration failed to load, so no
listeners where created.

Since the latest change on the mworker mode, HAProxy switch to wait mode
after successfuly loading the configuration, without cleaning its
listeners, because it was done in mworker_loop, resulting in the master
not closing its listeners and keeping them. The master needs its
configuration to know which listeners it need to close, so that must be
done before the exec().

This patch fixes the problem by cleaning the listeners in the
mworker_reexec() function.

No backport needeed.