Refactor ZC-specific staking functionality.

[Zannick]

In preparation for defining a RingCTStakeInput type, move more zerocoin-specific stuff behind a CStakeInput interface.

Namely:

• Getting the weight of the stake coin.
• Finishing up the transaction creation (RingCT may have more to do than zerocoin here).

Also moved a variable nCredit that needs to be reset on each loop into the loop.

Not tested beyond compiling.

[WetOne]

utACK bfbef4f