add an updates_timeout option to apt::params

Sometimes you can have a decent network connection and still exceed the
default timeout due to a large number of ppa's or other repositories, latency
somewhere out of your control, latency somewhere *in* your control that you
don't know about, etc, etc.  Allowing people to set this helps them manage
environments where they *don't* have decent connectivity and do use a number
of external repos.