ld.so.8: Note some further details of secure-execution mode

ld.so.8: Note some further details of secure-execution mode

Note some further details of the treatment of environment
variables in secure execution mode. In particular (as noted by
Matthias Hertel), note that ignored environment variables are also
stripped from the environment. Furthermore, there are some other
variables, not used by the dynamic linker itself, that are also
treated in this way (see the glibc source file
sysdeps/generic/unsecvars.h).

Reported-by: Matthias Hertel <Matthias.Hertel@rohde-schwarz.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>