Call mod_init when invoking state.single

This ensures that the state's ``mod_init()`` function is run when state.single is invoked.
``salt.state.State.call_chunk()`` is what runs ``mod_init()``, but ``call_chunk()`` is
not invoked by ``state.single`` since requisites do not need to be resolved.

This commit adds a call to ``salt.state.State._mod_init()`` just before
``salt.state.State.call()`` is invoked, making sure that the state's
``mod_init()`` function is called like it is in ``state.sls`` and
``state.highstate``.