util/HugeAllocator: move MADV_DONTFORK setting to HugeForkCow()

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

src/input/AsyncInputStream.cxx

@@ -38,7 +38,10 @@ AsyncInputStream::AsyncInputStream(EventLoop &event_loop, const char *_url,
 	 deferred_seek(event_loop, BIND_THIS_METHOD(DeferredSeek)),
 	 allocation(_buffer_size),
 	 buffer((uint8_t *)allocation.get(), _buffer_size),
-	 resume_at(_resume_at) {}
+	 resume_at(_resume_at)
+{
+	allocation.ForkCow(false);
+}
 
 AsyncInputStream::~AsyncInputStream()
 {

src/input/ThreadInputStream.cxx

@@ -53,6 +53,8 @@ ThreadInputStream::Start()
 	void *p = HugeAllocate(buffer_size);
 	assert(p != nullptr);
 
+	HugeForkCow(p, buffer_size, false);
+
 	buffer = new CircularBuffer<uint8_t>((uint8_t *)p, buffer_size);
 	thread.Start();
 }