chroot: don't chdir() if not changing root

This allows chroot to be used as a light weight tool
to change user identification for a command,
while not changing the current working directory.
It also makes `chroot / true` consistently succeed on
all platforms for non root users.

* src/chroot.c (main): If the same root is specified. i.e. '/'
then don't change the current working directory, and avoid the
overhead of the other redundant calls.
* tests/misc/chroot-fail.sh: Remove failure guard previously
needed on some systems.  Also add an explicit case to ensure
we don't change directory.
* NEWS: Mention the change in behavior.