• 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.