Wrap struct msgid_ds as a Go struct and add Stat() call

This exposes the C land ```struct msgid_ds``` (see ```msgctl(2)```), i.e., the result of ```msgctl(..., IPC_STAT)```, as a Go struct.

Tests pass on Darwin and Linux.

Little weirdness: The C struct type has a different name on Linux for some reason (no ```_new``` suffix). No idea what's up with that :/

@Sirupsen @burke / cc @wvanbergen