Rework default route handling.

[jkroonza]

1. replacedefaultroute option is being removed.
2. default route will now always be appended at defaultroute-metric (both IPv4 and IPv6) if defaultroute{.6} is given.

Closes: #473

[jkroonza]

There is also this reference to replacedefaultroute in pppd/ipcp.c in ipcp_clear_addrs:

2055     /* If replacedefaultroute, sifdefaultroute will be called soon
2056      * with replacedefaultroute set and that will overwrite the current
2057      * default route. This is the case only when doing demand, otherwise
2058      * during demand, this cifdefaultroute would restore the old default
2059      * route which is not what we want in this case. In the non-demand
2060      * case, we'll delete the default route and restore the old if there
2061      * is one saved by an sifdefaultroute with replacedefaultroute.
2062      */

I reckon this means further code adjustments may be required around there.

[jkroonza]

Need to add o_uint option.

[jkroonza]

By default iproute2 uses RTPROT_BOOT, somehow that doesn't seem right, but with this we get:

default *proto unspec* scope link metric 10000 

strace from ip ro ad shows (reformatted for readability):

sendmsg(3, {
  msg_name={
    sa_family=AF_NETLINK,
    nl_pid=0,
    nl_groups=00000000
  },
  msg_namelen=12,
  msg_iov=[
    {iov_base=[
      {
        nlmsg_len=44,
        nlmsg_type=RTM_NEWROUTE,
        nlmsg_flags=NLM_F_REQUEST|NLM_F_ACK|NLM_F_CREATE|NLM_F_APPEND,
        nlmsg_seq=1736279032,
        nlmsg_pid=0
      },
      {
        rtm_family=AF_INET,
        rtm_dst_len=0,
        rtm_src_len=0,
        rtm_tos=0,
        rtm_table=RT_TABLE_MAIN,
        **rtm_protocol=RTPROT_BOOT**,
        rtm_scope=RT_SCOPE_LINK,
        rtm_type=RTN_UNICAST,
        rtm_flags=0
      },
      [
        [{nla_len=8, nla_type=RTA_PRIORITY}, 10000],
        [{nla_len=8, nla_type=RTA_OIF}, if_nametoindex("ppp0")]
      ]
    ],
    iov_len=44
  }],
  msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 44

[Neustradamus]

@paulusmack, @sthibaul: Can you look this @jkroonza PR which solves one ticket :)