Misc: Cleanup across multiple backends

Remove the clear_transfer_priv() function from all backends besides
Linux. This function is only needed if the backend calls
usbi_handle_disconnect(), which only Linux does.

Remove the {attach,detach}_kernel_driver() functions from the Darwin
backend. They return LIBUSB_ERROR_NOT_SUPPORTED, but the same result is
achieved by having those functions be NULL.

Remove the init() and exit() functions from the SunOS backend. They are
optional and as no-ops are pointless.

Remove NULL and 0 initializers from usbi_backend structures.

Use named initializers in the NetBSD backend.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>