Configuration sections for testnet and regtest
----------------------------------------------

It is now possible for a single configuration file to set different
options for different networks. This is done by using sections or by
prefixing the option with the network, such as:

    main.uacomment=bitcoin
    test.uacomment=bitcoin-testnet
    regtest.uacomment=regtest
    [main]
    mempoolsize=300
    [test]
    mempoolsize=100
    [regtest]
    mempoolsize=20

The `addnode=`, `connect=`, `port=`, `bind=`, `rpcport=`, `rpcbind=`
and `wallet=` options will only apply to mainnet when specified in the
configuration file, unless a network is specified.

Add config changes to release notes
Add config changes to release notes
