CNeutrinoApp: add hook for neutrino start

Script will be executed on start of neutrino before. Handling is similar
like all other possible hooks, see global.h.

src/neutrino.cpp

@@ -2143,6 +2143,10 @@ void wake_up( bool &wakeup)
 
 int CNeutrinoApp::run(int argc, char **argv)
 {
+	puts("[neutrino] executing " NEUTRINO_APP_START_SCRIPT ".");
+	if (my_system(NEUTRINO_APP_START_SCRIPT) != 0)
+		perror(NEUTRINO_APP_START_SCRIPT " failed");
+
 	CmdParser(argc, argv);
 
 TIMER_START();