Add permissions to `wasi_config_preopen_dir` C API

[ajalt]

The current wasi_config_preopen_dir function does not expose the dir_perms and file_perms parameters that were added to preopened_dir. This commit adds them and updates the docs for those functions to reflect the current signature.

This is a prerequisite for https://github.com/bytecodealliance/wasmtime-py/issues/251

This change isn't backwards compatible, so please let me know if there's anything I need to do differently.