resolved: support libidn2 in addition to libidn

libidn2 2.0.0 supports IDNA2008, in contrast to libidn which supports IDNA2003.

https://bugzilla.redhat.com/show_bug.cgi?id=1449145
From that bug report:

Internationalized domain names exist for quite some time (IDNA2003), although
the protocols describing them have evolved in an incompatible way (IDNA2008).
These incompatibilities will prevent applications written for IDNA2003 to
access certain problematic domain names defined with IDNA2008, e.g., faß.de is
translated to domain xn--fa-hia.de with IDNA2008, while in IDNA2003 it is
translated to fass.de domain. That not only causes incompatibility problems,
but may be used as an attack vector to redirect users to different web sites.

v2:
- keep libidn support
- require libidn2 >= 2.0.0
v3:
- keep dns_name_apply_idna caller dumb, and keep the #ifdefs inside of the
  function.
- use both ±IDN and ±IDN2 in the version string