[GNC] Reconciliation flag revision

[Dale Alspach]

I have not switched to the 3.x version so I have no experience with the
implementation. However I find the notion of unreconciling
a transaction to be strange. Reconciliation is normally about matching a
group of transaction to an external document such as a bank or credit card
statement. Changing the flag from y to n of one or more transactions breaks
the reconciliation. What is the casual user supposed to do at this point?

At minimum the user should be given the option of preserving the
reconciliation flag. Popping up a warning puts the user on notice that
there may be a material change but the user is responsible for deciding
this and what to do about it.

Dale