MFC r279154,r279397:

r279154 (by jilles):

nice(): Correct return value and [EPERM] error.

PR: 189821
Obtained from: NetBSD
Relnotes: yes

r279397 (by jilles):

nice(): Put back old return value, keeping [EPERM] error.

Commit r279154 changed the API and ABI significantly, and {NZERO} is still
wrong.

Also, preserve errno on success instead of setting it to 0.

PR: 189821
Relnotes: yes