ControlPort now in 'next' branch

Part of the work at the latest GR Hackfest was to move ControlPort into gnuradio-core and get it ready to be merged into our 'next' branch (which is the upcoming 3.7 release). One of the main reasons why we wanted to put ControlPort into the core of the project was so that anything and everything in GNU Radio could make use of it. The start was to access parts of gr_top_block, the main component of the flowgraph. With (hopefully) a few tweaks, we expect to be able to remotely access the start/stop functionality of the top block, which gives us a whole new layer of control over GNU Radio applications.

The main reason that we didn't put ControlPort into gnuradio-core originally was that it was much easier to build as a conditional and optional component if it was its own separate gr-ctrlport.  But that meant that gr-ctrlport depended on gnuradio-core. We couldn't then also make gnuradio-core use gr-ctrlport, so nothing in gnuradio-core would have been able to use ControlPort functionality. Most of this would have changed come the complete switch over to the 3.7 structure and API where gr-ctrlport would only depend on gnuradio-runtime. But we discussed it and figured that allowing ControlPort options in things like gr_top_block is important, so we had no choice but to move it into gnuradio-core (and in 3.7 it will be in gnuradio-runtime).

The downside of this move was purely personal. I now had to work through the build system to have parts of gnuradio-core built if gr-ctrlport was enabled. But we did it, and now ControlPort is usable on all blocks and in any component. For anyone working off 'next', you now have complete access to ControlPort. Documentation, though, is still forthcoming...