BUG/MINOR: mworker: does not erase the pidfile upon reload

BUG/MINOR: mworker: does not erase the pidfile upon reload

When started in master-worker mode combined with daemon mode, HAProxy
will open() with O_TRUNC the pidfile when switching to wait mode.

In 2.5, it happens  everytime after trying to load the configuration,
since we switch to wait mode.

In previous version this happens upon a failure of the configuration
loading.

Fixes bug #1545.

Must be backported in every supported branches.

(cherry picked from commit 7b820a6191736cdf4167d624b7bcab9957dd0697)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>