Make sysctl_kern_proc_umask execute fast path when requested pid in
curproc->p_pid or 0, avoiding unnecessary locking. Update libc consumer
to skip calling getpid().

Submitted by: Pawel Biernacki <pawel.biernacki@gmail.com>
Reviewed by: mjg, robak
Approved by: mjg
Sponsored by: Mysterious Code Ltd.
Differential Revision: D12972