add constants for shared (GUI/core) -par settings

- introduce DEFAULT_SCRIPTCHECK_THREADS and MAX_HW_THREADS in main.h
- only show values from -MAX_HW_THREADS up to 16 for -par, as it
  makes no sense to try to leave more "cores free" than the system
  supports anyway
- use the new constants as min and max in the optionsdialog and remove
  defaults from .ui file

2nd commit makes use of MAX_HW_THREADS in miner.cpp.