Bug #197797 - Do not actively clear o+rx permission bits

Bug #197797 - Do not actively clear o+rx permission bits
on the /var/lib/portage directory. Only clear the o+w
bits. This allows users who aren't in the portage group
to have read access to the world file by default, but
administrators can explicitly chmod o-rx that directory
if necessary, and portage will leave those bits alone.

This makes the permission settings in dblink.treewalk()
consistent with those that are already defined in
config._init_dirs().

svn path=/main/trunk/; revision=8408