The error was due to casting `const` away for permuting argv
elements, which is necessary for a libc for compatibility with older
systems, but not for us.
Checkpatch will complain about the style not being followed, but this is not
our code, so it can be ignored.
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.
