zdtm/net/sysctl: add all to test and fix order

1. First save/check all,default,lo together and only after - set them,
as setting some sysctls can affect the others
2. Set "accept_redirects" after "forwarding" both for ipv4 and ipv6
3. Change order to "all", "default", "lo"

*One can not set ipv6.conf.all.stable_secret, so don't even try, see:
commit 622c81d57b39 ("ipv6: generation of stable privacy addresses for
link-local and autoconf")

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>