Improve resilience against memory attacks

To reduce residual fragments of secret data in memory after
deallocation, this patch replaces the global delete operator with a
version that zeros out previously allocated memory. It makes use of
the new C++14 sized deallocation, but provides an unsized fallback
with platform-specific size deductions.

This change is only a minor mitigation and cannot protect against
buffer reallocations by the operating system or non-C++ libraries.
Thus, we still cannot guarantee all memory to be wiped after free.

As a further improvement, this patch uses libgcrypt and libsodium
to write long-lived master key component hashes into a secure
memory area and wipe it afterwards.

The patch also fixes compiler flags not being set properly on macOS.

src/core/Alloc.cpp

@@ -0,0 +1,89 @@
+/*
+ *  Copyright (C) 2019 KeePassXC Team <team@keepassxc.org>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 2 or (at your option)
+ *  version 3 of the License.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <QtGlobal>
+#include <cstdint>
+#include <sodium.h>
+#ifdef Q_OS_MACOS
+#include <malloc/malloc.h>
+#else
+#include <malloc.h>
+#endif
+
+#if defined(NDEBUG) && !defined(__cpp_sized_deallocation)
+#warning "KeePassXC is being compiled without sized deallocation support. Deletes may be slow."
+#endif
+
+/**
+ * Custom sized delete operator which securely zeroes out allocated
+ * memory before freeing it (requires C++14 sized deallocation support).
+ */
+void operator delete(void* ptr, std::size_t size) noexcept
+{
+    if (!ptr) {
+        return;
+    }
+
+    sodium_memzero(ptr, size);
+    std::free(ptr);
+}
+
+void operator delete[](void* ptr, std::size_t size) noexcept
+{
+    ::operator delete(ptr, size);
+}
+
+/**
+ * Custom delete operator which securely zeroes out
+ * allocated memory before freeing it.
+ */
+void operator delete(void* ptr) noexcept
+{
+    if (!ptr) {
+        return;
+    }
+
+#if defined(Q_OS_WIN)
+    ::operator delete(ptr, _msize(ptr));
+#elif defined(Q_OS_MACOS)
+    ::operator delete(ptr, malloc_size(ptr));
+#elif defined(Q_OS_UNIX)
+    ::operator delete(ptr, malloc_usable_size(ptr));
+#else
+    // whatever OS this is, give up and simply free stuff
+    std::free(ptr);
+#endif
+}
+
+void operator delete[](void* ptr) noexcept
+{
+    ::operator delete(ptr);
+}
+
+/**
+ * Custom insecure delete operator that does not zero out memory before
+ * freeing a buffer. Can be used for better performance.
+ */
+void operator delete(void* ptr, bool) noexcept
+{
+    std::free(ptr);
+}
+
+void operator delete[](void* ptr, bool) noexcept
+{
+    ::operator delete(ptr, false);
+}