renice: support -- as per POSIX.

Sponsored by: Klara, Inc.
MFC after: 1 week

(cherry picked from commit e24c5c60d724120d137f9e5db4e59cf67fd06ba2)

renice: fix argument order.

The target modifiers (-g, -p, -u) may occur in any position except
between -n and its argument; furthermore, we support both the old
absolute form (without -n) and the modern relative form (with -n).

Sponsored by: Klara, Inc.

(cherry picked from commit 65ee0a8495538bcf462480c83a9a5dd6f23b6f95)

renice: add unit tests.

Sponsored by: Klara, Inc.

(cherry picked from commit 7ccdad905f30e90606e11104d46e39c9a6405279)