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.

If you don't feel invalidating the translations is worth it, let me know and I'll back that part of the change out.