core/msg: use disable/restoreIRQ

* needed to change internal `msg_send` to allow external disabling of interrupts

~~**Disclaimer:**
This is not beautiful, but I thought it's better than the status quo and might serve as a starting point for discussion.~~
Yep, this error was introduced with the application of  https://github.com/RIOT-OS/RIOT/pull/1887#issuecomment-62749973 ... addressed now.