Display tx nLockTime correctly when set to block #

Previously when a transaction was set to lock at a specific block the
calculation was reversed, returning a negative number. This broke the UI
and caused it to display %n in place of the actual number.

In addition the previous calculation would display "Open for 0 blocks"
when the block height was such that the next block created would
finalize the transaction. Inserted the word "more" and changed the
calculation so that the last message would be "Open for 1 more block" to
better match user expectations.
Merge pull request #2159 from petertodd/display-nlocktime-correctly

Display tx nLockTime correctly when set to block #