Create a class for deferred error logging (#390)

## Link to GitHub Issue, if one exists
#388 

## Description of change
Instead of setting flags in `signal` and logging when we crash, create a
class for logging deferred semi-fatal messages, and dump them to the log
immediately before crashing or shutting down.

## Testing
Tested each failure case by injection.

src/spice2x/util/deferlog.cpp

@@ -0,0 +1,44 @@
+#include <mutex>
+
+#include "deferlog.h"
+#include "util/logging.h"
+
+namespace deferredlogs {
+
+    const std::initializer_list<std::string> SUPERSTEP_SOUND_ERROR_MESSAGE = {
+        "audio initialization error was previously detected during boot!",
+        "    (W:SuperstepSound: Audio device is not available!!!)",
+        "    this crash is most likely related to audio init failure",
+        "    see if the default audio device changed, fix your audio configuration (e.g., sample rate)",
+        "    double check your spice audio options/patches, and try again"
+    };
+
+    std::vector<std::vector<std::string>> deferred_errors;
+
+    void defer_error_messages(std::initializer_list<std::string> messages) {
+        deferred_errors.emplace_back(messages);
+    }
+
+    void dump_to_logger() {
+        static std::once_flag printed;
+        std::call_once(printed, []() {
+            if (!deferred_errors.empty()) {
+                log_warning("deferred_error",
+                    "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
+                log_warning("deferred_error", "");
+            }
+
+            for (auto messages : deferred_errors) {
+                for (auto message : messages) {
+                    log_warning("deferred_error", "{}", message);
+                }
+                log_warning("deferred_error", "");
+            }
+
+            if (!deferred_errors.empty()) {
+                log_warning("deferred_error",
+                    "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
+            }
+        });
+    }
+}