MAC/do: 'struct rule': IDs and types as 'u_int', rename fields

[olce]

Summary

This revision is part of a series. Click on the Stack tab below to see the context.
This series has also been squeezed into D47633 to provide an overall view.

Commit message:
This is in preparation for introducing a common conversion function for
IDs and to simplify code a bit by removing the from-IDs union and not
having to introduce a new one for to-IDs in a later commit.

[olce]

sys/security/mac_do/mac_do.c

47–48

TODO: Re-reading this code, I think (int)(uid_t)-1 == -1 doesn't match my intent as it will always be considered true by any decent compiler on 2-complement machines and consequently cannot detect if uid_t is effectively a signed type.

[olce]

Use u_int instead of int, as uid_t and gid_t are in fact unsigned integers, and not signed ones as I initially assumed. (Since we are compiling with -fwrapv, this doesn't matter for correctness of execution, but is conceptually wrong.)