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. 
May be copy it to `core.internal.traits`? It contains some duplicates, which should be used inside druntime.