Replace idnkit-1 support with idnkit-2 support (or drop it)

Currently, BIND doesn't compile with idnkit-2:

host.c:20:10: fatal error: 'idn/result.h' file not found
#include <idn/result.h>
         ^~~~~~~~~~~~~~
dighost.c:30:10: fatal error: 'idn/result.h' file not found
#include <idn/result.h>
         ^~~~~~~~~~~~~~
1 error generated.

and for IDNA2008 support, we need to either add support for idnkit-2 or drop idnkit-1 support and leave only libidn2 support (!56 (closed)). The only reason why to keep idnkit-2 would be the licensing. idnkit-2 is licensed under custom JPRS BSD-like (with additional restrictions) license, and libidn2 is LGPL3+ (for our purposes) licensed.

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