wallet: cancellable refresh

^C while in manual refresh will cancel the refresh, since that's
often an annoying thing to have to wait for. Also, a manual refresh
command will interrupt any running background refresh and take
over, rather than wait for the background refresh to be done, and
look to be hanging.

src/common/util.cpp

@@ -48,7 +48,7 @@ using namespace epee;
 
 namespace tools
 {
-  std::function<void(void)> signal_handler::m_handler; 
+  std::function<void(int)> signal_handler::m_handler;
 
 #ifdef WIN32
   std::string get_windows_version_display_string()