util: move util/message to common/signmessage

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.

src/test/util_tests.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <clientversion.h>
+#include <common/signmessage.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC
 #include <hash.h> // For Hash()
 #include <key.h>  // For CKey
 #include <sync.h>
@@ -12,7 +13,6 @@
 #include <util/bitdeque.h>
 #include <util/fs.h>
 #include <util/fs_helpers.h>
-#include <util/message.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC
 #include <util/moneystr.h>
 #include <util/overflow.h>
 #include <util/readwritefile.h>