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. 
I wouldn't be disinclined to move it to object.d, but that belongs in a different pull.
Simply converting them here manually seems good enough.