add fits_movnam_hdu()

Wrap the function `fits_movnam_hdu()`.

I also threw in a small change to `show()` whereby the current HDU is indicated with an asterisk.

While implementing this I noticed that none of the statuses are getting captured in ccall. Passing the pointer `&f.status` in ccall doesn't allow the c library to update `f.status`, so `f.status` always remains 0, even when a non-zero status is returned by cfitsio. I'm going to work on fixing this in a separate PR.