New twiddle 'xmbean' command for producing skeleton xmbean descriptors

Writing xmbean descriptors for existing (standard) mbeans is tedious to do by hand.

Introduce a new command for twiddle, called 'xmbean' that prints out mbean metadata as an xmbean descriptor,

Simply redirect the output and use it as a skeleton for the xmbean descriptor.

It can be used as a debugging aid, too, that prints out the detailed MBeanInfo information for a live MBean.

Usage: xmbean <object-name>