POSIX_SPAWN_SETPGROUP et al should be of type `c_short`

[godmar]

The constants POSIX_SPAWN_* are for the __flags field in posix_spawnattr_t and are set via posix_spawnattr_setflags, both of which use c_short.

Should they be of type c_short as well?

[tgross35]

I was thinking this was a case like #1619, where the struct takes a short but the signature takes an int. However, this seems to be a case where the function and the struct are both short - are there any counterexamples?

If not, we can do this for 1.0.