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/file_access_unix.cpp

@@ -354,14 +354,7 @@ FileAccess *FileAccessUnix::create_libc() {
 
 CloseNotificationFunc FileAccessUnix::close_notification_func = nullptr;
 
-FileAccessUnix::FileAccessUnix() :
-		f(nullptr),
-		flags(0),
-		last_error(OK) {
-}
-
 FileAccessUnix::~FileAccessUnix() {
-
 	close();
 }