lib: util: move fnmatch to lib/posix since its a posix function

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
Just perusing over what is in lib/util, I happened to find fnmatch(). Had never heard of it before, so checked if there was a manual page.

As it so happens, there is! And it's a POSIX function.
https://pubs.opengroup.org/onlinepubs/9699919799/functions/fnmatch.html
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fnmatch.h.html

Describe the solution you'd like
Should therefore be moved to the POSIX library.

Describe alternatives you've considered

Additional context
Will be part of v3.3 POSIX Roadmap

[cfriedt]

Additionally, this function has no test coverage, so please add some tests for it.