Fix some issues with nice(). SUSv3 specifies that it return the new

Fix some issues with nice().  SUSv3 specifies that it return the new
priority if successful, while the linux syscall returns 0.  Also, in
the non-syscall version, we must be sure to set errno to EPERM on failure
since setpriority() can set errno to several possible values.
Note: The version of glibc on my debian system is broken wrt EPERM too.