8c (for real)

- new do_plug() and add_plug() functions
- plugs work differently to hooks
    - plugs extend functions, so they expect some kind of return value
    - hooks replace functions
- new sys-includes/form.php
    - can write form text fields, select fields and buttons
    - no organised form <-> field association yet
- moved the db connect into a sys-includes/db.php file dedicated to
database functions