Since introduction for support of ip6-privacy (use_tempaddr,
RFC4941) with commit d376270bfe,
the sysctl value from /etc was always read first.
This is problematic, because an explicit setting in the
connection should not be ignored over a global configuration.
Drop that old behavior. It was also problematic, because we did
not read any files under /etc/sysctl.d (except for sysctl.conf).
Also, we did not honor per-interface configurations.
Now we also use as last fallback the value from
/proc/sys/net/ipv6/conf/default/use_tempaddr
That has the advantage of falling back to the system default value
so that NM doesn't need to have it's own default policy
(Related: https://bugzilla.redhat.com/show_bug.cgi?id=1187525).
This is a change in behavior.
f3c61f8141