zebra: include MTU option in RA on request (BZ#665)

This implements a new "ipv6 nd mtu <1-65535>" interface-level command.

* doc/ipv6.texi: add description
* zebra/rtadv.c
  * rtadv_send_packet(): send option type 5, when configured
  * ipv6_nd_mtu(): new VTY helper
  * no_ipv6_nd_mtu(): ditto
  * rtadv_config_write(): add new option
  * rtadv_init(): list new helpers
2 files changed