This backports elements of Core PR11403:
- Support P2WPKH addresses in create/addmultisig : https://github.com/bitcoin/bitcoin/pull/11403/commits/37c03d3e05713e14458ca2867e442926f165afea
- Support P2WPKH and P2SH-P2WPKH in dumpprivkey : https://github.com/bitcoin/bitcoin/pull/11403/commits/cf2c0b6f5cde584d7004ff1b5b476ef54de6b74b