libc/stdio: Add stdio file locking functions

[maht]

Summary

Add flockfile(), ftrylockfile() and funlockfile() functions [1] when the CONFIG_STDIO_DISABLE_BUFFERING is not set.

[1] POSIX.1-2008 / System Interfaces / flockfile https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/functions/flockfile.html

Impact

Extending current interfaces (backward compatible).

Testing

Probably some tests are needed to be added to incubator-nuttx-apps.