Simplify pm_send_validate and pass custom fields/values through.

By changing "$message = array();" to "$message = $form_state['values']" we can..

a) Simplify the code a bit, since we don't need to set every element manually, only those that need manual handling.

b) It allows to add custom fields to the form and use these values inside the hooks, especially hook_privatemsg_message_insert, since that is the only one that exposes the mid while a custom submit function doesn't.

- This is the same behavior as the node system does use, with the exception that $node is an object, see: http://api.drupal.org/api/function/node_submit/6

- It is already possible when you use the api, as all passed in $options are added to $message.

Thoughts?