Idna module supplying xlat

This module provides IDNA encoding for internationalized domain names via libidn11.

It's not very useful on its own but will be needed for DDDS.

It's a separate module rather than an rlm_expr due to the library dependency -- I doubt anyone will want to maintain an "in-house" implementation of IDNA, it's ugly.

Please let me know what else should be done to this tree if anything is lacking.  Or wrong.