Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

core/io/packet_peer_dtls.h

@@ -60,7 +60,7 @@ public:
 	static PacketPeerDTLS *create();
 	static bool is_available();
 
-	PacketPeerDTLS();
+	PacketPeerDTLS() {}
 };
 
 VARIANT_ENUM_CAST(PacketPeerDTLS::Status);