Minor update to signed module file format

[Sean Mullan]

I have made a very small change to the signed module file format [1]. The size 
of the signatureLength field has been changed to u4 (from u2) in order to 
accomodate signatures whose size may exceed this boundary (for example, due to 
large certificate chains, etc).

We have started the implementation and should be ready to review and putback our 
first set of changes in the next week or so. Therefore, if you are interested, 
but haven't yet reviewed the format and specification [2], please try to review 
and send your comments soon.

Thanks,
Sean

[1]: http://cr.openjdk.java.net/~mullan/jigsaw/signed-module-file-format
[2]: http://cr.openjdk.java.net/~mullan/jigsaw/signed-module-functional-spec