[IPV4] Fix ip=dhcp regression

David Brownell pointed out a regression in my recent "Fix ip command
line processing" patch. It turns out to be a fairly blatant oversight on
my part whereby ic_enable is never set, and thus autoconfiguration is
never enabled. Clearly my testing was broken :-(

The solution that I have is to set ic_enable to 1 if we hit
ip_auto_config_setup(), which basically means that autoconfiguration is
activated unless told otherwise. I then flip ic_enable to 0 if ip=off,
ip=none, ip=::::::off or ip=::::::none using ic_proto_name();

The incremental patch is below, let me know if a non-incremental version
is prepared, as I did as for the original patch to be reverted pending a
fix.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>