Introduce setrlimit_uid(2)

[bnovkov]

Summary

This change introduces setrlimit_uid(2), a new system call for changing
per-UID resource limits.

Unlike the setrlimit(2) system call, setrlimit_uid(2) operates on the
plimit structure stored in struct uidinfo.

[brooks]

This makes me wonder why not setrlimit_type(id_t id, int /*idtype_t*/ type, int which, const struct rlimit *rlp), but it's not clear most types really want limits so it feels overall like premature generalization.

sys/kern/syscalls.master

3428

sys/sys/resource.h

200

I feel like I'd put the uid_t at the front of the list so it's "subject", "action", but don't have a strong justification.

[bnovkov]

Rebase and address @brooks 's comment.