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/kernel/context.cpp

@@ -14,12 +14,8 @@
 
 
 namespace kernel {
-Context* g_context;
-
 Context::Context()
 {
-    assert(!g_context);
-    g_context = this;
     std::string sha256_algo = SHA256AutoDetect();
     LogPrintf("Using the '%s' SHA256 implementation\n", sha256_algo);
     RandomInit();
@@ -29,8 +25,6 @@ Context::Context()
 Context::~Context()
 {
     ECC_Stop();
-    assert(g_context);
-    g_context = nullptr;
 }
 
 } // namespace kernel