PIMv6 test snapshot, 2022-04-30

primary test target: PIMv6 SSM-only + MLD

* MRIB support is broken.  Best to use only URIB for RPF lookups
  (`ip multicast rpf-lookup-mode urib-only` in zebra), otherwise
  inconsistent RPF lookup results may be used (this will NOT result in a
  clear and obvious error, the inconsistencies may be very hard to
  debug!)
  This also affects IPv4; to address this a PIM NHT rework is in
  progress but that will take some time still.

* deleting an interface or VRF can crash the pim daemon (in both IPv4
  and IPv6.)  PR 10706 included in this testpoint partially fixes this,
  but the fix is incomplete.  The issue is not IPv6 specific and being
  worked on on FRR master.

* interface routes from sources other than "(C)onnected" are not
  understood as directly connected by pimd, and result in pimd failing
  RPF lookup since it expects a PIM neighbor for "non-connected"
  sources.  Do not use e.g.  static routes to mark additional IPv6
  subnets as on-link, instead configure an address in that subnet to get
  a `C>` route.
  (This issue also affects IPv4, but the adding routes for connected
  subnets without configuring an address is more common for IPv6.)

* SSM ranges cannot be changed from the default `ff3x::/32` yet.  Use
  multicast groups in this range for SSM testing.

* MLD is enabled with `ipv6 pim sm`.  The `ipv6 mld` option does
  nothing.  The YANG model also has some inconsistencies regarding the
  time units on MLD timers (deciseconds vs. milliseconds.)  The best way
  to approach this is still under discussion.

* all debug flags are unconditionally enabled.  This may result in
  excessive log disk space usage.  Configuring minimum level for log
  targets still works correctly, i.e. "info" targets won't log debug
  messages.

* PIM Register messages and bootstrap support are included in this test
  snapshot because they are necessary for ANVL testing.  The primary
  focus of this testpoint is SSM;  all ASM features may still contain
  significant bugs.

* do not rely long-term on `pim6d.conf` to configure pim6d.  Reading
  that file has been enabled to aid testing, but since this is a new
  daemon, and integrated config is supposed to be the way to go, pim6d
  will only support that.  As a quick helper, to reload a single
  daemon's integrated config, use:
    `vtysh -b -d pim6d`
  This is also the reason pim6d does not have a telnet port by default.
  The `-A` and `-P` daemon command line options will remain available to
  manually enable a telnet port.