This patch removes the home-brewed resizeable arrays used to store
altsetting structures, along with the now-unneeded max_altsetting field.
Since we are already making a preliminary pass through all the descriptors
to check their lengths, we take the opportunity to also count the number
of altsetting descriptors for each interface.  Then exactly the right
number can be allocated all at once.

This also moves the code that allocates the altsettings outside the
usb_parse_interface() routine.  Though not important now, this change will
come in handy in the next patch.