Allow custom form state to be passed to entity_get_form()

Unlike drupal_get_form(), entity_get_form() does not pass along any other arguments given to it.
That has led to the Views UI attaching arbitrary data to the entity before passing it along, which is not ideal.
Contrib will very likely need to do this as well.

In order to implement this, I also had to clean up the arguments passed to entity_get_form() and entity_form_state_defaults(). entity_get_form() now mirrors entity_submit_form(), and entity_form_state_defaults() is less tailored to one use case.