Support cloning the clingo Control

[teiesti]

Following a few discussions from last week (@sthiele, @MaxOstrowski, and @wanko), I'd like to suggest support for cloning the clingo Control.

Such a feature would be helpful whenever multiple but different solving task should be performed that share a common beginning. E.g., remember the @susuhahnml's approach to incremental configuration. Using externals, she had to go to a great length to disable newly-grounded facts for parts that where later removed. Even though, such patterns might not be entirely avoidable, having the change to snapshot the current clingo Control, might at least give the opportunity to perform a clean-up from time to time.

@rkaminsk suggested forking the process. While that is probably the easiest and most-efficient solution, it only works for Unix systems and does not play well with other APIs because one is immediately forced to deal with interprocess communications.