lxc: give RW access to /proc/sys/net/ipv[46] to containers

lxc: give RW access to /proc/sys/net/ipv[46] to containers

Some programs want to change some values for the network interfaces
configuration in /proc/sys/net/ipv[46] folders. Giving RW access on them
allows wicked to work on openSUSE 13.2+.

Reusing the lxcNeedNetworkNamespace function to tell
lxcContainerMountBasicFS if the netns is disabled. When no netns is
set up, then we don't mount the /proc/sys/net/ipv[46] folder RW as
these would provide full access to the host NICs config.