Small tweaks

- JtR initializes only requested devices from now on.
- How many contexts we are going to use is an open question at this time.
- I suppose you have hardware that sometimes fails during initialization. If so, please test 4aa0aef. Well, if you request some device(s) and all of them fail, the program should exit nicely (line 418 in common-opencl). Otherwise, it should work without problems using (only) the working requested device(s) without warnings.
- To make pwsafe follow the guidelines, i put new_keys there. Now, it only misses split kernel. (BTW: i tried one of my real passwords using md5crypt-opencl and it ASIC hang my hardware.).