xdrgen: XDR width for union types

Not yet complete.

The tool doesn't do any math yet. Thus, even though the maximum XDR
width of a union is the width of the union enumerator plus the width
of its largest arm, we're using the sum of all the elements of the
union for the moment.

This means that buffer size requirements are overestimated, and that
the generated maxsize macro cannot yet be used for determining data
element alignment in the XDR buffer.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>