zebra: justify rtadv VTY commands with arguments

zebra: justify rtadv VTY commands with arguments

ipv6 nd ra-interval
ipv6 nd ra-lifetime
ipv6 nd reachable-time
ipv6 nd home-agent-preference
ipv6 nd home-agent-lifetime
ipv6 nd router-preference

Calls to atoi() and atol() are replaced with VTY_GET_INTEGER_RANGE()
macro, command patterns are clarified and aliases of some commands
are added for consistency. Other changes are listed below.

* zebra/rtadv.c
  * ipv6_nd_ra_interval_msec(): resolve -Wsign-compare
  * ipv6_nd_ra_interval(): idem
  * rtadv_init(): update to list new aliases
* doc/ipv6.texi: update to match current implementation