nice(3) doesn't return new priority
The nice(3) library call returns 0/-1, whereas it should return the new priority
according to POSIX expectations.
I attach a patch to fix this. The nice(1) utility isn't affected in any way
whatsoever, since it doesn't use the nice(3) call. It uses get/setpriority()
If no one objects, I'll commit it in the next days.