bridge: Change bridge's 'ports' member from array to hmap.

bridge: Change bridge's 'ports' member from array to hmap.

In my opinion, this makes the code more obviously correct in many places,
because there are generally fewer variables.  One must generally keep two
variables in sync for iterating through an array: the array index and the
contents of the array at that index.  For iterating through an hmap, only
the map element is necessary.

A linked list would also be a reasonable choice for the bridge's collection
of ports.  I chose to use an hmap because we already had an index by name
and it seemed OK to use only one index.  I decided not to keep the shash
because they are less convenient for iteration than an hmap.