Fix #882: active state of interfaces depends on ipv6

Fix #882: active state of interfaces depends on ipv6

read_etc_network_interfaces used the content of
/proc/net/if_inet6 to decide whether an interface's state is
"active", which means an interface is only active when it
has an ipv6 address, thus using net.ipv6.conf.*.disable_ipv6
on an interface will cause it to show as inactive in the web
interface.

We now filter the interfaces found in /proc/dev/net by their
IFF_UP flag read via an SIOCGIFFLAGS ioctl().