Update: Add f_id_t, f_uid_t, and f_gid_t type helpers for uid_t and gid_t.

Update: Add f_id_t, f_uid_t, and f_gid_t type helpers for uid_t and gid_t.

The `uid_t` and `gid_t` don't readily map to each other.

I would love to simplify the code and have a single function call in several cases when using both `uid_t` and `gid_t`.

This provides interchangeable types `f_id_t`, `f_uid_t`, and `f_gid_t`.
The standard behavior is that `f_uid_t` and `f_gid_t` are both sub-types of `f_id_t`.
The `f_id_t` is by default an unsigned 32-bit integer.