Get rid of shutdown.cpp/shutdown.h, use SignalInterrupt directly

This change is mostly a refectoring that removes some code and gets rid of an
unnecessary layer of indirection after #27861

But it is not a pure refactoring since StartShutdown, AbortShutdown, and
WaitForShutdown functions used to abort on failure, and the replacement code
logs or returns errors instead.

src/test/util/setup_common.cpp

@@ -40,7 +40,6 @@
 #include <rpc/server.h>
 #include <scheduler.h>
 #include <script/sigcache.h>
-#include <shutdown.h>
 #include <streams.h>
 #include <test/util/net.h>
 #include <test/util/random.h>