mcd v3.0

MLD querier support. mcd now runs one querier per address family on
every enabled interface, so the Linux bridge gets a real querier for
IPv6 too.

Note: MLD is on by default on every enabled interface, use
mld off to keep an interface IGMP only. mctl show interfaces
has a new Proto column and its JSON output lists both families.

Changes

• Add MLD querier, RFC 2710 and RFC 3810. Queries are sent from the
  link-local address of the interface, with its own election, MLDv2
  to MLDv1 fallback, and the same startup query burst as IGMP
• New iface keywords mldv1, mldv2, igmp on|off and mld on|off
• Add static-group per interface, IPv4 and IPv6. The group is
  joined in the kernel, which answers queries for it like any host, so
  a bridge keeps a permanent host membership for it in the MDB. The
  memberships survive SIGHUP
• mctl show interfaces lists both queriers per interface with a new
  Proto column. JSON output gains a family key per querier and a
  top-level mld-queriers list in show igmp json
• Link up and down now reach every entry of an interface, raw VLAN
  entries used to miss them
• Drop unused --with-systemd configure option, no unit file was ever
  installed
• Add man pages mcd(8), mcd.conf(5) and mctl(8), also available at
  https://man.troglobit.com
• Rewritten README with figures and worked examples

Fixes

• Fix use-after-free when an interface goes down with active group
  timers or a remote querier being tracked
• Fix potential list corruption when the same interface is configured
  with more than one vlan
• Fix mctl writing terminal probe escape sequences to stderr when
  stdout is not a terminal, e.g. when piped to grep