Organize RPCCommands table

Use sensible categories (overall control, P2P, blockchain/UTXO and
mining, wallet, wallet-enabled mining) and sort within each.

Also remove unnecessary #ifdef ENABLE_WALLET from `rpcnet.cpp`.

Functionality-neutral change.
Merge pull request #3964

ab88ed9 Organize RPCCommands table (Wladimir J. van der Laan)