nproxy: Add missing chdir("/") after chroot()

Also make sure that fd 0,1,2 are correctly replaced with /dev/null.
As we (might) chroot() before daemonizing, we must open /dev/null before
chroot-ing.

Reported-By: Morten Stevens
Merge pull request #970 from zeha/nproxy-chroot-chdir

nproxy: Add missing chdir("/") after chroot()