Add's sigfillset and sigdelset which will be used in D76676.
Not a blocker but just something which struck my mind as I was reading the POSIX docs again today. POSIX doesn't seem to care about set == nullptr. We could still detect an invalid set and set a different error may be so that we don't step on POSIX's way of setting EINVAL for a bad signum. May be EFAULT? Whatever we do, we need to do it consistently across all of these related functions. Since sigaddset already has similar code, we can keep it as is here for now.