Kernel: Fix kernel null deref on process crash during join_thread()

The join_thread() syscall is not supposed to be interruptible by
signals, but it was. And since the process death mechanism piggybacked
on signal interrupts, it was possible to interrupt a pthread_join() by
killing the process that was doing it, leading to confusing due to some
assumptions being made by Thread::finalize() for threads that have a
pending joiner.

This patch fixes the issue by making "interrupted by death" a distinct
block result separate from "interrupted by signal". Then we handle that
state in join_thread() and tidy things up so that thread finalization
doesn't get confused by the pending joiner being gone.

Test: Tests/Kernel/null-deref-crash-during-pthread_join.cpp

Tests/Kernel/null-deref-crash-during-pthread_join.cpp

@@ -0,0 +1,21 @@
+#include <pthread.h>
+#include <stdio.h>
+#include <sys/select.h>
+#include <unistd.h>
+
+int main(int, char**)
+{
+    pthread_t tid;
+    pthread_create(
+        &tid, nullptr, [](void*) -> void* {
+            sleep(1);
+            asm volatile("ud2");
+            return nullptr;
+        },
+        nullptr);
+
+    pthread_join(tid, nullptr);
+
+    printf("ok\n");
+    return 0;
+}