Merge #10825: net: set regtest JSON-RPC port to 18443 to avoid conflict with testnet 18332

Summary:
ce3baa193 changed regtest RPCport to 18443 to avoid conflict with testnet 18332 (Ferdinando M. Ametrano)

Pull request description:

using the same JSON-RPC default port for both testnet and regtest prevents running both at the same time on the same machine. Since RPCport=P2Pport-1 for both mainnet and testnet, and regtest P2Pport being 18444, 18443 is proposed for regtest RPCport

Documentation has been updated (or created where missing); manpages doc/man/bitcoin*.1 could include information for regtest too

Tree-SHA512: d42185f7ef54dc918ece19b543c8681d08bb9c5a971394e21f2d9a1091734b091b08df69fab622c207b46f402cf9323ded5b7a33fbd0af722388930169124e7f

Backport of Core PR10825
https://github.com/bitcoin/bitcoin/pull/10825/files

Test Plan:

make check
test_runner.py

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3773