Update PRoot changelog.
Prepare release of PRoot v4.0.0 (2/2).
When executed by the root user, skip tests that assume no privileges.

Reported-by: Rémi Duraffort <remi.duraffort@st.com>
Fix support for accessat(2) and fchmodat(2): they have only 3 parameters, not 4.

For faccessat(2) and fchmodat(2) syscalls, the fourth parameter is
*not* used by the kernel, only the libc uses it.  As a consequence,
PRoot shall ignore this flag.

This bug was exposed by the following command:

    proot -S stage3-amd64-hardened+nomultilib-20140710 emerge util-linux

when Gentoo's sandbox is enabled.