The raw syscall(SYS_fchmodat2, ...) in DirFd::chmod_at bypassed LD_PRELOAD
tools (fakeroot, fakechroot, pseudo), so a recursive mode change landed on
disk but their bookkeeping never saw it, silently and with exit status 0.
Try libc fchmodat() first and keep the raw syscall as a fallback for the
NoFollow case libc declines; glibc issues fchmodat2 from there anyway.
Add util/check-libc-interposition.sh, which runs chown -R and chmod -R
under fakeroot and compares what fakeroot reports afterwards.
Fixes#14028
Sylvestre Ledru