zebra: justify some IPv6 ND RA timers wrt RFC

There was a regression introduced with the previous commit:
"ipv6 nd home-agent-lifetime 1800000" appeared by default in every
interface section of running-config, although this  command is
invalid in this context. Troubleshooting and bugfixing of the issue
tracked out several bugs in router advertisement procedures, some of
which are fixed in this commit.

* zebra/interface.c
  * if_zebra_new_hook(): update to treat -1 as "uninitialized"
  * nd_dump_vty(): idem
* zebra/rtadv.c
  * rtadv_send_packet(): update processing of "router lifetime" field,
    "home agent" option and "home agent lifetime" field to conform to
    RFC6275 better
  * ipv6_nd_ra_interval_msec(): update MaxRtrAdvInterval range check,
    make sure it never exceeds (initialized) AdvDefaultLifetime
  * ipv6_nd_ra_interval(): idem
  * ipv6_nd_ra_lifetime(): update AdvDefaultLifetime range check, make
    sure it never falls below MaxRtrAdvInterval
  * ipv6_nd_homeagent_lifetime(): update HomeAgentLifetime range check
  * no_ipv6_nd_ra_lifetime(): update to treat -1 as "uninitialized"
  * no_ipv6_nd_homeagent_lifetime(): idem
  * rtadv_config_write(): idem

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