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/stream_peer_tcp.cpp

@@ -362,10 +362,7 @@ void StreamPeerTCP::_bind_methods() {
 }
 
 StreamPeerTCP::StreamPeerTCP() :
-		_sock(Ref<NetSocket>(NetSocket::create())),
-		timeout(0),
-		status(STATUS_NONE),
-		peer_port(0) {
+		_sock(Ref<NetSocket>(NetSocket::create())) {
 }
 
 StreamPeerTCP::~StreamPeerTCP() {