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.

platform/android/net_socket_android.cpp

@@ -72,11 +72,6 @@ void NetSocketAndroid::make_default() {
 	_create = _create_func;
 }
 
-NetSocketAndroid::NetSocketAndroid() :
-		wants_broadcast(false),
-		multicast_groups(0) {
-}
-
 NetSocketAndroid::~NetSocketAndroid() {
 	close();
 }