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_udp.cpp

@@ -343,12 +343,6 @@ void PacketPeerUDP::_bind_methods() {
 }
 
 PacketPeerUDP::PacketPeerUDP() :
-		packet_port(0),
-		queue_count(0),
-		peer_port(0),
-		connected(false),
-		blocking(true),
-		broadcast(false),
 		_sock(Ref<NetSocket>(NetSocket::create())) {
 	rb.resize(16);
 }