BUG/MEDIUM: mworker/httpclient: initialization skipped by accident in mworker mode

BUG/MEDIUM: mworker/httpclient: initialization skipped by accident in mworker mode

Since commit fe75c1e12da061 ("MEDIUM: startup: remove
MODE_MWORKER_WAIT") the MODE_MWORKER_WAIT constant disappearded. The
initialization of the httpclient proxy was conditionned by this
constant.

The proxy must be created in mworker mode, but only in the worker not in
the master. It was currently completely disabled in both the master and
the worker provoking a NULL dereference upon httpclient usage.

No backport needed.