Add support for libidn2 (IDNA 2008 + TR46)

Please review and feel free to merge / open a new branch.

Test testpkcs11.sh fails here, so I can't test everything.

$ cat testpkcs11.log 
Testing PKCS11 support
usage: ./testpkcs11.sh: [pkcs15|softhsm|sc-hsm]
FAIL testpkcs11.sh (exit status: 1)