rc.local: add option to disable ipv6
rc.local: default "accept router advertisements" config to "no"

when the router sends "router advertisement" [0] packets,
linux will set up ipv6 routes automatically, which has the effect
that tests (connect() on udp socket) musl's getaddrinfo does
(to determine which addresses to prefer) succeed in the ipv6 case and
therefore return the ipv6 addresses first.
if the router has no external ipv6 address programs like wget will hang for
a long time until falling back to the ipv4 address.

test program to examine musl's dns lookup behaviour via strace or gdb:
(set breakpoint to qsort and look at the callers frame)

 #include <sys/socket.h>
 #include <netdb.h>
 #include <stdio.h>

int main() {
        struct addrinfo hint = { .ai_socktype = 1 };
        struct addrinfo *res;
        int ret = getaddrinfo("matt.ucc.asn.au", 0, &hint, &res);
        if(!ret) {
                if(res->ai_family == AF_INET) puts("got ipv4 first");
                if(res->ai_family == AF_INET6) puts("got ipv6 first");
        }
        return ret;
}

anything that happens automatically without explicit request
regarding network configuration is considered broken.

more info: [1]

[0] http://packetlife.net/blog/2008/aug/28/ipv6-neighbor-discovery/
[1] http://strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/