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.

drivers/unix/net_socket_posix.cpp

@@ -173,9 +173,7 @@ void NetSocketPosix::cleanup() {
 }
 
 NetSocketPosix::NetSocketPosix() :
-		_sock(SOCK_EMPTY),
-		_ip_type(IP::TYPE_NONE),
-		_is_stream(false) {
+		_sock(SOCK_EMPTY) {
 }
 
 NetSocketPosix::~NetSocketPosix() {