posix: fd_mgmt: implement `dup()`

[cfriedt]

This is part of the See POSIX Roadmap for LTSv3 (RFC #51211).

dup() is required as part of POSIX_FD_MGMT Subprofiling Option Group.

For more information, please refer to https://pubs.opengroup.org/onlinepubs/9699919799/functions/dup.html