Add Multi-User and POSIX File Permission Support to Apache NuttX

This project aims to introduce a POSIX-style multi-user permission model in Apache NuttX. Currently, NuttX does not fully associate user identity with processes or enforce file access permissions based on ownership and mode bits, which limits POSIX compatibility and structured access control. The proposed approach integrates user credentials (uid, gid) into task groups, extends inode metadata to include ownership and permission bits, and enforces access control through the Virtual File System (VFS) layer. Permission checks will follow POSIX semantics and be applied consistently across filesystem operations and device nodes. In protected and kernel builds, permission enforcement will be strict due to kernel mediation. In flat builds, while strict enforcement is not possible, the system will ensure consistent permission checks along standard VFS paths and restrict updates to permission metadata through defined interfaces. The project will deliver a credential management system, extended filesystem metadata, VFS-level permission enforcement, system call interfaces such as getuid, setuid, chmod, and chown, along with testing and documentation.