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

- it belongs to parameter-to-internal flags step, not parameter
  interactions phase
Bitcoin-Qt: fixes for using display unit from options

- extend PaymentServer with setOptionsModel() and rework initNetManager()
  to make use of that
- fix all other places in the code to use display unit from options and no
  hard-coded unit
Merge pull request #2940 from Diapolo/checkpoint

move Checkpoints:fEnabled from step 2 to step 3 in init
Merge pull request #2930 from Diapolo/displayUnit

Bitcoin-Qt: fixes for using display unit from options