jpkg enhancements to create signed modules

[Wang Weijun]

>
>   Why is the --nosign option needed?  Not signing a module should be the
>   default; signing should be done only when requested.
>

I thought signing with the single privateKeyEntry using JKS type 
~/.keystore is the default when no signature-related option is provided. If 
default action is no-sign, it looks a --sign option is needed, since all 
other options are optional.

Max