Both `getopt_long` and `getopt_long_only` are GNU-specific, so we'll be
copying musl's implementation for portability.
This was retrieved as of commit 90251cf73dfdd44e7a3f085d236e89a7dff1b00b.
musl is licensed as MIT, which is compatible (being identical...) to RGBDS'.
The file is being copied as-is, without a copyright notice or attribution,
but this is only to have a verbatim copy in the history. Those will be added
in the next commit.
