Move util/message to common/signmessage so it is named more clearly, and
because the util library is not supposed to depend on other libraries besides
the crypto library. The signmessage functions use CKey, CPubKey, PKHash, and
DecodeDestination functions in the consensus and common libraries.
This is required for removing the UniValue copy constructor.
-BEGIN VERIFY SCRIPT-
sed -i 's/return NullUniValue/return UniValue::VNULL/g' $(git grep -l NullUniValue ':(exclude)src/univalue')
-END VERIFY SCRIPT-
