MFC r279154,r279397:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@313479 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f