Everywhere: Move global Kernel pattern code to Kernel/Library directory

This has KString, KBuffer, DoubleBuffer, KBufferBuilder, IOWindow,
UserOrKernelBuffer and ScopedCritical classes being moved to the
Kernel/Library subdirectory.

Also, move the panic and assertions handling code to that directory.

Kernel/Library/UserOrKernelBuffer.cpp

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Library/UserOrKernelBuffer.h>
+#include <Kernel/Memory/MemoryManager.h>
+
+namespace Kernel {
+
+bool UserOrKernelBuffer::is_kernel_buffer() const
+{
+    return !Memory::is_user_address(VirtualAddress(m_buffer));
+}
+
+ErrorOr<NonnullOwnPtr<KString>> UserOrKernelBuffer::try_copy_into_kstring(size_t size) const
+{
+    if (!m_buffer)
+        return EINVAL;
+    if (Memory::is_user_address(VirtualAddress(m_buffer))) {
+        char* buffer;
+        auto kstring = TRY(KString::try_create_uninitialized(size, buffer));
+        TRY(copy_from_user(buffer, m_buffer, size));
+        return kstring;
+    }
+
+    return KString::try_create(ReadonlyBytes { m_buffer, size });
+}
+
+ErrorOr<void> UserOrKernelBuffer::write(void const* src, size_t offset, size_t len)
+{
+    if (!m_buffer)
+        return EFAULT;
+
+    if (Memory::is_user_address(VirtualAddress(m_buffer)))
+        return copy_to_user(m_buffer + offset, src, len);
+
+    memcpy(m_buffer + offset, src, len);
+    return {};
+}
+
+ErrorOr<void> UserOrKernelBuffer::read(void* dest, size_t offset, size_t len) const
+{
+    if (!m_buffer)
+        return EFAULT;
+
+    if (Memory::is_user_address(VirtualAddress(m_buffer)))
+        return copy_from_user(dest, m_buffer + offset, len);
+
+    memcpy(dest, m_buffer + offset, len);
+    return {};
+}
+
+ErrorOr<void> UserOrKernelBuffer::memset(int value, size_t offset, size_t len)
+{
+    if (!m_buffer)
+        return EFAULT;
+
+    if (Memory::is_user_address(VirtualAddress(m_buffer)))
+        return memset_user(m_buffer + offset, value, len);
+
+    ::memset(m_buffer + offset, value, len);
+    return {};
+}
+
+}