[QT] Don't ask for a passphrase to getnewaddress.

With an encrypted wallet the GUI was prompting for a passphrase every time
 the user requested a new address.  This is unnecessary, increases the
 exposure to keyboard sniffers, and discourages using fresh addresses for
 every transaction.

Instead only prompt for a passphrase when the keypool runs out, also call
 the new address function with the flag that prevents reuse.

Thanks to AlexNagy on IRC for pointing this out and who wouldn't take any
 lip from a curmudgeonly developer and insisted on what he knew to be true.
Remove fAllowReuse from GetKeyFromPool.

With the GUI password fix this was always false.
Bitcoin-Qt: add testnet check and icon to intro dialog
move Checkpoints:fEnabled from step 2 to step 3 in init

- it belongs to parameter-to-internal flags step, not parameter
  interactions phase
Merge pull request #2904 from gmaxwell/newaddr-no-passphrase

[QT] Don't ask for a passphrase to getnewaddress.
Merge pull request #2931 from Diapolo/intro

Bitcoin-Qt: add testnet check and icon to intro dialog
Merge pull request #2940 from Diapolo/checkpoint

move Checkpoints:fEnabled from step 2 to step 3 in init