- changes importprivkey behavior to overwrite existent label if one
is passed and keep existing ones if no label is passed
- tests behavior of importprivkey on existing address labels and
different same key destination
This is a backport of Core PR13381 .
I had to modify heavily the last test to create a P2SH manually. Bitcoin Core does produce a segwit wrapped into P2SH address by default so this isn't a problem for them.