No new functionality, just a continuation of my work toward completing #665.
I removed the common members of `struct withdrawal` and `struct fund_channel`
and placed them in a new `struct wallet_tx`. Then it was fairly straightforward
to reimplement the existing code in terms of `wallet_tx`.
Since I made some structural changes I wanted to get this approved before I
go any farther.
Added 'all' to fundchannel help message.
