LibC: Improve error handling for unimplemented functions

[po-nuvai]

This PR improves error handling for several unimplemented LibC functions:

• getpriority()/setpriority(): Set errno to ENOSYS before returning -1
• chroot(): Set errno to ENOSYS before returning -1
• nice(): Set errno to ENOSYS and return -1 (was incorrectly returning the incr argument)
• fnmatch(): Return FNM_NOMATCH instead of 0 (which falsely indicates a match)

These functions now properly indicate they are not implemented by setting errno to ENOSYS, following POSIX conventions.

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[spholz]

I would keep these FIXME dbglns. Having this output makes it easier to debug ports that attempt to use these functions and fail because we don't implement them yet.

[oskar-skog]

According to POSIX, fnmatch should return a non-zero value other than FNM_NOMATCH to indicate an error.
https://pubs.opengroup.org/onlinepubs/9799919799/functions/fnmatch.html

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!