Represent Bug Status Values as Sets

The conversions between different systems of status values are ambiguous. In
many cases the values of certain status fields are undefined, or several values
are possible for a status field.

Therefore the status values should be represented with a set of possible
values, with a preferred or default value.

Before a merged bug is uploaded, it is compared to the cached version of
itself. If the value of the cached bug is in the set of the merged
bug, the two values are considered equal, and the value from the cached bug is
used as the default value.

Before uploading the bug, the set is exchanged by its default value.

See:
bug #949820