This patch touches the most significant places in usbcore
that need to respond to configuration change events:

  - set_interface changes altsettings; affects endpoints
    in both new and old settings (but not other endpoints).

  - set_configuration; affects every endpoint

The endpoints get disabled, flushing hardware state
reliably.  This resolves two FIXMEs here, part of one
in OHCI (ed_get) and all of one in EHCI (qh_append_tds).

Not many drivers currently use these code paths, but
now they should start to work properly.