sysctl: fix compatibility with procps sysctl

Busybox sysctl is incompatible with procps when '.' appears in
directory name, mostly happens with VLANs.

        busybox syntax (since 2008): net.ipv4.conf.eth0.100.mc_forwarding
         procps syntax (since 2002): net.ipv4.conf.eth0/100.mc_forwarding
                 (supported by both: net/ipv4/conf/eth0.100/mc_forwarding)

Use procps syntax for output; for input, allow both.

function                                             old     new   delta
sysctl_dots_to_slashes                                86     143     +57
sysctl_act_on_setting                                443     453     +10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 67/0)               Total: 67 bytes

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>