Multi-users support and Privilegies separation on NuttX

NuttX currently supports functions related to multi-users like chown, chmod, etc, however we don't have yet multi-user support neither privileges separation for users (even in Protected or Kernel mode).

It is something that will bring more security to NuttX when using in environments were critical applications needs to run and the board /equipment needs to have communication with external world.

Also Role-Based Access Control (RBAC) it something that will be important to have after multi-user and privileges separation.

Refence to RBAC on Unix: https://dl.acm.org/doi/pdf/10.1145/319171.319180