Use std::io::Error for error results, except get_planar_framebuffer

[ids1024]

Based on discussion on #173. Which this supersedes.

All functions in drm and drm_ffi returning SystemError, except get_planar_frambuffer, only use it for representing standard Unix error kinds. So std::io::Error can be used instead.

For get_planar_framebuffer, a GetPlanarFramebufferError error is defined. It might be nice if Rust has anonymous sum types for this sort of thing, but this seems like the typical pattern. And better than using the same error kind everywhere if UnrecognizedFourcc isn't possible in any other function.

This should remove nix from the public API of the crate. This makes errors a bit simpler to handle for users of the library using rustix or a different version of nix, and allows drm to change which it uses without a breaking API change.

Testing this in Smithay (Smithay/smithay@master...ids1024:smithay:drm-rs-update), the section where it was matching on several variants of SystemError is simplified, and other things aren't changed much. Using io::Error in a variant did however mean removing Clone and Copy from drm_lease::Error. But those shouldn't really be needed for an error kind generally.

(Softbuffer is also trivial to update. cosmic-comp required no changes with the patched Smithay above. I'm not familiar with other users of drm-rs.)

In some ways the Errno types from nix or rustix would be better, but it's best not to have a public dependency on that or duplicate it.