Actually sends the PM, assuming there weren't any errors

• Postback action for the form loaded by MessagePost()
• Censors form data as needed
• Checks for errors and calls messagePostError() if it finds any
• Checks for a valid session ID
• Handles previewing the PM (if $_REQUEST['preview'] is set)
• Parses out the recipients and makes sure they're valid
• Sends message to all valid recipients
• Redirects back to action=pm