Merge #11626: rpc: Make logging RPC public

Summary:
cabff7588 rpc: Make logging RPC public (Wladimir J. van der Laan)

Pull request description:

This started out as a developer hack but now it's useful enough for general use. Unhide the call by moving it to `control` category. This makes it documented in `help`.

Tree-SHA512: f45fa378558b552d4e2a110bf85100b0eaaa6180bb5f62cb54a251f66026d4625b670c69d85c281eebbf4b56b80b65618c51a5a593b8f9d0a04b31e95adc91f4

Backport of Core PR11626
https://github.com/bitcoin/bitcoin/pull/11626/

Depends on D3464

Test Plan:

make check
./bitcoind
./bitcoin-cli help

The logging rpc should appear under the Control section.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3684