Rework IDN support in dig

IDN support in dig currently suffers from a number of issues:

IDNA2003 fallbacks are still present, despite our test comments claiming that BIND made a hard transition to IDNA2008 non-transitional processing,
confusing (passing flags which are ignored by the callee) and fragile (locale-dependent) libidn2 calls are used when decoding Punycode found in upstream DNS responses,
leftover Autoconf macros from previous IDN implementations are still present,
redundant code is present.