When specified in bitcoin.conf without using the [regtest] or [test]
section header, or a "regtest." or "test." prefix, the "addnode",
"connect", "port", "bind", "rpcport", "rpcbind", and "wallet" settings
will only be applied when running on mainnet.

When network-specific options such as -addnode, -connect, etc are
specified in the default section of the config file, but that setting is
ignored due to testnet or regtest being in use, and it is not overridden
by either a command line option or a setting in the [regtest] or [test]
section of the config file, a warning is added to the log, eg:

Warning: Config setting for -connect only applied on regtest network

when in [regtest] section.

Partial backport of core PR11862
Commits d1fc4d9, 68797e2 and 608415d

Depends on D2434

make check

echo "rpcport=10042" > ~/.bitcoin/bitcoin.conf
./src/bitcoind -regtest

Check in ~/.bitcoin/regtest/debug.log that the warning is logged