Add setpriority(2)/getpriority(2)

setpriority(2) is useful for limiting the impact of processes running in
a sandbox. It is also possible to exec the sandbox process using nice:

    alcove:execvp(Drv, Path, "/usr/bin/nice", ["/usr/bin/nice", "-n",
            "19", "sandbox"]).

The advantage of supporting set/getpriority natively in alcove are:

* nice doesn't need to exist in the sandbox chroot
* better error values (nice will return errors as a binary string)
* priorities can apply to fork trees