Kernel: Remove unneeded Thread::set_default_signal_dispositions

The `default_signal_action(u8 signal)` function already has the
full mapping. The only caveat being that now we need to make
sure the thread constructor and clear_signals() method do the work
of resetting the m_signal_action_data array, instead or relying on
the previous logic in set_default_signal_dispositions.

Kernel/Syscalls/execve.cpp

@@ -544,7 +544,6 @@ KResult Process::do_exec(NonnullRefPtr<FileDescription> main_program_description
     m_coredump_metadata.clear();
 
     auto current_thread = Thread::current();
-    current_thread->set_default_signal_dispositions();
     current_thread->clear_signals();
 
     clear_futex_queues_on_exec();

Kernel/Thread.cpp

@@ -84,7 +84,7 @@ Thread::Thread(NonnullRefPtr<Process> process, NonnullOwnPtr<Region> kernel_stac
     }
     if constexpr (THREAD_DEBUG)
         dbgln("Created new thread {}({}:{})", m_process->name(), m_process->pid().value(), m_tid.value());
-    set_default_signal_dispositions();
+
     m_fpu_state = (FPUState*)kmalloc_aligned<16>(sizeof(FPUState));
     reset_fpu_state();
     m_tss.iomapbase = sizeof(TSS32);
@@ -547,6 +547,9 @@ void Thread::clear_signals()
     m_signal_mask = 0;
     m_pending_signals = 0;
     m_have_any_unmasked_pending_signals.store(false, AK::memory_order_release);
+
+    Span<SignalActionData> action_data(m_signal_action_data);
+    action_data.fill({});
 }
 
 // Certain exceptions, such as SIGSEGV and SIGILL, put a
@@ -842,14 +845,6 @@ DispatchSignalResult Thread::dispatch_signal(u8 signal)
     return DispatchSignalResult::Continue;
 }
 
-void Thread::set_default_signal_dispositions()
-{
-    // FIXME: Set up all the right default actions. See signal(7).
-    memset(&m_signal_action_data, 0, sizeof(m_signal_action_data));
-    m_signal_action_data[SIGCHLD].handler_or_sigaction = VirtualAddress(SIG_IGN);
-    m_signal_action_data[SIGWINCH].handler_or_sigaction = VirtualAddress(SIG_IGN);
-}
-
 RegisterState& Thread::get_register_dump_from_stack()
 {
     auto* trap = current_trap();

Kernel/Thread.h

@@ -1007,8 +1007,6 @@ public:
 
     FPUState& fpu_state() { return *m_fpu_state; }
 
-    void set_default_signal_dispositions();
-
     KResult make_thread_specific_region(Badge<Process>);
 
     unsigned syscall_count() const { return m_syscall_count; }
@@ -1241,7 +1239,7 @@ private:
     u32 m_kernel_stack_top { 0 };
     OwnPtr<Region> m_kernel_stack_region;
     VirtualAddress m_thread_specific_data;
-    SignalActionData m_signal_action_data[32];
+    SignalActionData m_signal_action_data[32] { };
     Blocker* m_blocker { nullptr };
 
 #if LOCK_DEBUG