Add 'about' information to -version output

Adds a copyright and attribution message to the `-version` output
(the same as shown in the About dialog in the GUI).

Move the message to a function LicenseInfo in init.cpp.

src/init.h

@@ -28,6 +28,9 @@ enum HelpMessageMode
     HMM_BITCOIN_QT
 };
 
+/** Help for options shared between UI and daemon (for -help) */
 std::string HelpMessage(HelpMessageMode mode);
+/** Returns licensing information (for -version) */
+std::string LicenseInfo();
 
 #endif