Commonly used network configuration tools write scoped IPv6 nameserver
addresses to /etc/resolv.conf.  libirs only handles these when it is
compiled with -DIRS_HAVE_SIN6_SCOPE_ID, which is not the default, and
only handles numeric scopes, which is not what network configuration
tools typically use.  This causes dig to be practically unable to handle
scoped IPv6 nameserver addresses in /etc/resolv.conf.

Fix the problem by:

  - not requiring a custom compile-time flag to be set in order for
    scoped IPv6 addresses to be processed by getaddrinfo(),

  - parsing non-numeric scope identifiers using if_nametoindex(),

  - setting the sin6_scope_id field in struct sockaddr_in6 structures
    returned by getaddrinfo() even if the AI_CANONNAME flag is not set.