Make pseudofs(9) create directory entries in order, instead
of the reverse.

This fixes Linux sysctl(8) binary - it assumes the first two
directory entries are always "." and "..". There might be other
Linux apps affected by this.

NB it might be a good idea to rewrite it using queue(3).

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21550