Network activity toggle

brings back @jonls #2412.
Rebased and overhauled:
- togglenetwork RPC command renamed to "setnetworkactive true|false"
- GUI toggling is now by pressing the connections icon in the status bar (it's a hidden feature now!)

In fact, NACK merging with this license. Section 4(c) seems to give the author special privileges that are non-free and could conceivably be problematic.