Make linux stat(2) return the same st_dev for every devfs instance.
The reason for this is to work around an idiosyncrasy of glibc
getttynam() implementation: it checks whether st_dev returned for
fd 0 is the same as st_dev returned for the target of /proc/self/fd/0
symlink, and with linux chroots having their own devfs instance,
the check will fail if you chroot into it.
This is somewhat horrible, but I don't see a better way to fix it.