Add conditional compilation about IPv6 sources and headers, change code style

Not Using RT_USING_LWIP_IPV6, it will not compile any sources and headers about IPv6.(The IPv6 headers have conditional compilation included).
CHANGELOG/init.c/netif.h/opt.h/test_dhcp.c is the change of the git head.
Sorry that I did not see the diff file too carefully in local computer, So I commit too many times