rpc: creates possibility to preserve labels on importprivkey

- 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

doc/release-notes-pr13381.md

@@ -0,0 +1,29 @@
+RPC importprivkey: new label behavior
+-------------------------------------
+
+Previously, `importprivkey` automatically added the default empty label
+("") to all addresses associated with the imported private key.  Now it
+defaults to using any existing label for those addresses.  For example:
+
+- Old behavior: you import a watch-only address with the label "cold
+  wallet".  Later, you import the corresponding private key using the
+  default settings.  The address's label is changed from "cold wallet"
+  to "".
+
+- New behavior: you import a watch-only address with the label "cold
+  wallet".  Later, you import the corresponding private key using the
+  default settings.  The address's label remains "cold wallet".
+
+In both the previous and current case, if you directly specify a label
+during the import, that label will override whatever previous label the
+addresses may have had.  Also in both cases, if none of the addresses
+previously had a label, they will still receive the default empty label
+("").  Examples:
+
+- You import a watch-only address with the label "temporary".  Later you
+  import the corresponding private key with the label "final".  The
+  address's label will be changed to "final".
+
+- You use the default settings to import a private key for an address that
+  was not previously in the wallet.  Its addresses will receive the default
+  empty label ("").