util: Add SignalInterrupt class and use in shutdown.cpp

This change helps generalize shutdown code so an interrupt can be
provided to libbitcoinkernel callers. This may also be useful to
eventually de-globalize all of the shutdown code.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/kernel/context.cpp

@@ -14,9 +14,12 @@
 
 
 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();
@@ -26,6 +29,8 @@ Context::Context()
 Context::~Context()
 {
     ECC_Stop();
+    assert(g_context);
+    g_context = nullptr;
 }
 
 } // namespace kernel