Msg.d

Added POSIX wrapper for <sys/msg.h>

I asked for help on this link: http://forum.dlang.org/thread/lradkwgmyotmldzhynrc@forum.dlang.org and was advised to do a pull request.  This is my first pull request. 
That structure is architecture dependent, see [here](https://sourceware.org/git/?p=glibc.git&a=search&h=HEAD&st=grep&s=msqid_ds).